I think the problem is that a lot of good selling exclusive games were on the PS2. Look here:
Game (Platform(s)): Total/Combined Sales (relative userbase at release of game)
GTA:SA (PS2): 15 million (80 million)
COD4 (360+PS3): 6.85 million (combined 24 millionish)
FFX (PS2): 7.95 million (20 millionish)
AC (PS3+360): 5.47 million (combined 24 millionish)
SMG (Wii): 5.63 million (around 14 million? Somewhere in there)
Halo 3 (360): 7.19 million (11 million)
The important thing to remember here is that during the PS2 era (and at the release of FFX in most regions), the PS2 was WAY ahead of the GC and Xbox (I believe when the GC and Xbox were launched, the PS2 already had 20 million+ users). So most of PS2s exclusives were obvious. Even if they launched for the other consoles, would enough GC owners buy FFX to make the estimated 10% "porting" cost worth it? Probably not, the userbase wasn't big enough. And the user bases never got big enough. And since each port requires a estimated 10% porting cost, a lot of games were simply not ported over because 40 million Xbox+GC owners weren't even enough to warrent that cost. That and with 3x the install base of Xbox+GC combined, the PS2 sold more than enough software. What's the difference between 15 million GTA:SA and 16 million? Not a lot.
But now we have a different situation. The 360 failed to gain the momentum and lead in it's first year compared to the PS2. So the 360 isn't the "ideal choice" for an exclusive, since the PS3 has half the userbase, and thus could make up a good 35% of sales (i.e.: 360 @ 16.8 million, PS3 @ 9.8 million, combined @ 26.6 million, PS3/combined = 35% of the total userbase, which should mean 35% of the total sales). So it's obvious that multiplats now a days sell better than exlusives do now a days. I won't say how things will end up, to stay on topic, but for now at least, multiplats probably sell better than exclusives, though that's not 100% the case.
To sum it up, for those who don't want to read: Well actually, 100% of the time multiplats will sell better than if it were exlusive, mainly because I'm sure at least 1 person would buy it on each consoles, thus ensuring at least 1 extra unit sold compared to if it were exclusive. The key thing to remember is, "is it worth the cost to get the extra units sold." Right now, it most definitely is. In a few years? Who knows. With the cost of games this generation, I can see how it might always be necessary to make games multiplat.
