Why is "Carry" a role at all?

The best way to compare a carry heroes is to see how they scale with items vs another hero thats not a carry.
Compare an Anti Mage 6 slotted vs the pub-star Spirit Breaker 6 slotted, both lvl 25..just watch the AM melt him.

It really semlls like troll, but let's give him a try:

1) DOTA is a team game, you're supposed to play as a team, or at least is usefull to suppose that the enemy team is playng alltogather, comparing 2 heroes in a 1vs1 fight is quite pointless IMHO (unless you're considering they're lane dominance, but that's another kind of discussion).

2) yes, most of hard carry are weaker in early game, most of them don't have anithing but their r-click to help in early teamfight, that's why you call them "weak".

3) if your 10 lvl sniper is not farmed, you've lost your lane, and sameone on the other will probably have quite a good farm, you will see the difference between a farmed carry and a non farmed one!

4) diversity between heroes is what make DOTA a very interesting and fun game, balancing evry heores would result in a quite boring game IMHO!

sry for my english!XD Have fun!

My definition of a "Carry" would be "a Hero that has Abilities that synergise well with Items".

Look at Phantom Lancer and Naga Siren, they both synergise immensely well from Diffusal Blade. On most heroes, Diffusal Blade's Passive (Feedback) only gives a mild damage increase, but since it passes to Illusions, this damage increase is multiplied by Phantom Lancer and Naga Siren's Illusions.

This, however, doesn't mean that all Carries need a ton of farm to be effective, e.g. "Semi-Carries".

Look at Juggernaut, with 1 point in Bladefury he can deal out 400 AoE DAMAGE at lvl 1, right from the start of the game! If you get Phase Boots, Drum of Endurance and/or Yasha on him, you can easily chase most opponents down for the full 700 damage, with 4 points in Bladefury. Of course there's more to Juggernaut than Bladefury, but with an item or 2, he can make greater use of it.