Multiple sequels for a single franchise in the same generation is a bad thing, not a good one. It exhausts the interest from the franchise, and takes away time from other games you could be making.
Take Naughty Dog, for instance. This is what they've done on consoles:
PS: Crash Bandicoot 1, 2, 3, Racing
PS2: Jak 1, 2, 3, Racing
PS3: Uncharted 1, 2, 3
They make good games overall, with 3 different franchises, but they've stuck to only one franchise per gen. Most people are tired after the second game, a third (per gen) is pushing it. Imagine if they instead made 1 Crash, 1 Jak, and 1 Uncharted per gen? They'd have 3 solid franchises still alive today.
Instead, Crash and Jak are dead, and people are getting tired of Uncharted. This isn't unique to Sony, of course, Microsoft had Bungie stuck on a single franchise for too long, and Gears of War is seeing too many releases. By attemping to "play it safe," these companies end up killing off their IP's, their future.
