If the game was longer or the greatest thing everybody has ever played then it might be worth it, but it's not long enough and has no replayability. Why should I pay double what I feel is worth it just because it's an indie game?
I'm getting sick of this mindset of "indie devs deserve more money simply because they're indie devs and they need support". No, they get my money when their game is worth it. You know what I bought at full price? Super Meat Boy. DeathSpank. THOSE are worth the price of admission. Limbo isn't because of its length (for me). It's too short and I'll only play it once if that. It's hard for me to drop $10 on Limbo when not too long ago I dropped about the same $ for Borderlands + all expansions. Price vs. enjoyment ratio is huge to me.