Many people I know (myself included) have multiple accounts. Perhaps even the majority of people I know. Many people I know (not including myself) have bought 2 or more boosts. Buying a boost on a new account allowed you to start over on a new faction without having to lose your characters on the other faction - if you didn’t want to pay for 2 subs, you could just let the other account’s sub lapse. It wasn’t good for faction population health, but the service was there, so might as well use it.
I’ve seen several people on these forums state that they rerolled from horde to alliance at the start of TBC in order to have better battleground queues (poor them). The boost made this easier to do. I can go dredge up the posts if you really want me to but I’d rather not.
From what I’ve heard and seen (anecdotally, yes), the majority of people that did change factions went the other way.
I’m sure you can. But for a lot of us, the price of a boost is worth a far, far less than 24 hours of our time, especially doing something we didn’t want to do, and thus their existence dramatically lowered the barrier to rerolling.