Well, yes, and no. The pro's can do it for several reasons:
1) They're really good.
2) Their team are all really good too.
3) They have excellent team communication and co-ordination.
4) Their strategy and team are setup to make those choices work.
If any of these things fall down...well you can end up in big trouble. And in the average pub game, you'll frankly be lucky to get to even number 1 of that list, let alone the rest.
This is generally the biggest issue with carry players - going for really greedy builds like Hand of Midas AND Battle Fury in the hope of having some uninterrupted farming time. If it works, their GPM will be off the charts and they'll win easily - although it's really their team that has done the hard work providing space to farm, or the opposition's for giving them that space!
Of course, if things don't work out then they're in big trouble as they have almost zero fighting ability with that build. The trick here is to be flexible - say your Anti-Mage already has a Hand of Midas and a Ring of Health towards your Battle Fury. If your team has already lost two towers and are down on kills, it could be a good idea to switch to the Vanguard build instead...
HOWEVER, what I really want to talk about is...
Supports! Yes, really. One of the biggest fails I routinely see on pub supports is them trying to emulate the pro builds with bare boots and rushed Mekansms. Force Staff and no stats, or in really bad cases, no wards.
I hold that basic stat items are very important for support heroes, especially in less co-ordinated games. Stats can keep you alive, or give you that vital bit of mana you need to get a crucial ability off.
This goes double when playing against heroes with high burst damage - the likes of Nyx Assassin, Lina, Tiny and many others. Often these heroes have vicious combos...but they almost always max out at a certain damage level - if you can get more HP than they can put out in one go, you'll usually survive and get away. This saves you gold and experience, while denying it to the opposition.
Does buying stats slow down your bigger items? Sometimes. A frequent problem for inexperienced supports trying to buy items like a Mekansm is trying to get 900 gold together without dying and falling back, never quite reaching it. However, even a Mekansm won't protect you if you fall victim to a stun/nuke combo that doesn't give you the chance to use it. It also leads to bad habits like not warding to try to save gold - again a poor investment in terms of your own survival and your team's success.
So what kind of items am I talking about? Generally pretty meagre ones like a Bracer, Urn of Shadows or Magic Wand. They're cheap, and come in easy to manage chunks of cash that allow you to keep buying essentials like Observer Wards, Dust of Appearance and Town Portal Scrolls around them. Even a Null Talisman, Ring of Basilius or Wraith Band can help, although they tend to less popular on supports.
Boots of Speed are obviously very important too - movement speed does make a difference to your positioning and survivability, but they aren't all you need against some line ups, or on some heroes. In fact, sometimes it can even be beneficial to get some stats together before your boots (Heresy!), although this is rare.
Extension items can also be quite important too. Many players simply go with whatever the cookie-cutter recommendations are without looking at what's in front of them. Here's a quick guide (mostly centered around intel supports, but applies to most):
- Mekansm. Great team item, but also offers you better armour and health regeneration. Burst heal can get you or allies out of trouble. Not great against magical burst damage as it only offers +250 hp if you get the chance to use it.
- Force Staff. Useful utility item, mainly used as an escape. Very useful when kiting, when near impassable terrain and dodging skill shots. Not useful versus burst damage, very mobile opponents.
- Ghost Scepter. Very useful versus physical damage, late game carries, especially heroes like Riki, Clinkz and Juggernaut. Provides some stats too. Worse than useless against magic damage. Often needs to be combined with a TP scroll to fully escape.
- Eul's Scepter of Divinity. Movement speed boost and either a self or enemy part disable. Great versus channelling spells (especially if you don't have a stun). Lets you dodge slower spells or wait out the effects of others. Good for setting up unreliable abilities. Less effective the more magic immunity they have.
- Drum of Endurance. Stats and speed boost - both offering increased survivability and an aura for your team. Hit the active for a further boost. Generally very useful, but not if another on your team has them already. Other items are better in specific situations.
- The humble Town Portal Scroll. Many players don't realise just how effective an escape they can be, especially combined with some of the above items. Heroes who don't have at least a mini-stun will have to knock off your entire HP pool in the few seconds it takes to channel a TP. If you use these at the right time, you can escape very effectively.
I'm frankly amazed by the number of supports these days who seem to ignore stats items. Yes, late game you're still food without utility stuff, but dying a whole bunch of times before then due to lack of stats is a poor way of solving this issue.
It's a bit like watching people trying to run while under the effect of Rupture/ Bloodrage when they're carrying a TP scroll.
Also, a small mistake: you claim that a Ghost Scepter "Often needs to be combined with a TP scroll to fully escape", but in fact the TP cancels your immunity to physical damage. You can't understand how many times I died because of that when I didn't know.
Mind you...
As you mention, combining them is a great idea - a friendly Astral Imprisonment could easily give you the full 400hp heal.
...it would only give you 200HP, the heal takes 8 seconds...
More strength than a bracer while mana regen and a heal as long as your team is ahead it helps sustain a push.
...and it provides the same Strength as a Bracer.
I think you missed blink since it gives any initiator or support good positioning without having to put themselves at risk. Plus you can escape if you have good reflexes.
As you mention, combining them is a great idea - a friendly Astral Imprisonment could easily give you the full 400hp heal. Combined with a Magic Wand these items can really help you turn a potential death into a "got away with almost no health". You still need to have a big enough HP pool to get the chance to use them tho!