=P Hmm let's see... for me, it's that you can beat your enemies... strategically! Exception is made when sometimes it's easy to mass some unit and hard to mass its counter, so the counter becomes useless. Such a situation happens in SC2 with immortals and stalkers. If your enemy is massing stalkers and you went for a Waygate build, too bad, you have to go for Stalkers too and micromanage: robotics is too expensive, economically and time-wise.
In Wc3's particular case, I like it that I don't have to expand to be useful in the game. Just a few weeks ago I played a team melee where my allies expanded and had gryphons and whatnot, but I was still the one holding off the enemy with only my Alchemist, tauren chieftain, 1 or 2 tauren and few headhunters. We eventually lost, as one of my allies decided he'd sulk and afk midway (these ridiculous situations still happen, yes =P), but it was a bloody awesome game. This is a detail I like in WC3: you can be useful to your team no matter how small your forces are.