yeah, i've been wanting Galaga Legions for a long time, but never wanted to pay full price. I got Namco Museum ($13.50 w/ coupon) for 3 reasons.
1) 30+ more games
2) 800 more gamer points
3) when I'm tired of it, I can sell/trade it for "something".
Number 3 is the main reason I don't like buying XBLA games. There aren't a lot of games I'll want to keep after I beat them, so I feel like I'm wasting my money if I can't resell or trade when I'm done.