I won't say that FFXIII didn't factor into my purchase of a PS3, because it did (got my PS3 in Feb of this year). Generally, all the consoles I get are for a FF game. The thing is, a FF is usually the catalyst for me. I'll be interested in a console for a while until a Final Fantasy shows up and convinces me, and then I'll go hogwild for those first few months buying everything that's caught my eye. My gaming tastes vary beyond FF, but for all its ups and downs, it's still my favourite.
The biggest thing for me, and probably for most people, is investment. Back when I got my PS2 ('03), I had no idea that it'd end up having probably my all-time favourite console gaming library, but I did go in with a a bit of (now justified) excitement for the future. For the time, it looked pretty appealing and had a pretty big backlog of games I wanted (many of which ended up disappointing), but it was long before the time of what, in hindsight, I consider to be the real PS2 system-sellers: GTA:San Andreas, God of War, Shadow of the Colossus, DQVIII, and FFXII. These games provided the experiences I had been wanting without even knowing it, and none of them were really in the news when I got the PS2.
I had the same thought process picking up my PS3 Slim this year, but as of this moment, I don't think the game library is really keeping up its end of the bargain. I've had my eye on PS3 for a while, and if four years after the release isn't enough time to show off some real muscle, who knows if it'll come at all. I've waited longer after the initial release to pick up the PS3 than I did the PS2, but it seems like the games aren't there. Granted, there are a few highly-rated PS3 exclusives, but it's still pretty sparse for this far into the console's lifecycle.
In short, a console being 'worth it' depends at what point in the lifecycle one is purchasing it. Early to mid-cycle is usually a couple of pretty good games + future promise. Those who only buy once the new generation is on the horizon are probably targeting a specific bunch of games. I fall into the former group, but it's much more hit-or-miss.