I've been playing since Vanilla, and the idea that everyone played one toon is bologna. The only people I knew that could get away with not having a healer/tank alt in major guilds were those already playing Tanks/healers. Granted most people didn't more than 1 or 2, but if you were hardcore, in many cases you were expected to roll one.
The only reason TBC seemed harder on alts was because the old Vanilla zones hadn't been adjusted yet and the attunments made it a pita. Once they dropped attunements you started seeing lots and lots more people playing on alts. Kara was an alt haven mid TBC.
Wrath and Cata opened the flood gates because things like leveling through PvP, zero attunements, easy heroics, and easy access to gear made them fun time wasters. LFR made it even more so.
MoP and all the rep grinds pretty much put a stopper on the alt parades, and I have leveled less alts this expansion than I did in TBC. Thats not all bad because I have found plenty to do on my mains, but when you have 10 85's you kind of want to get around to playing them some.