Why wouldn't you buy a bundle for just one game? Hell, as long as it's in 'tier 1' of that bundle (I really don't know what to call the 'tiers') you only have to pay like, $1/?0.50 for 4-6 games.
I dunno but I think that ?0.50 for 4-6 games just based off I had a slight interest in 1 of them is quite good. I've stumbled upon many good games this way I wouldn't have played otherwise...For just ?0.50. That's less than a soda and lasts 10 times longer.
I'm not saying "just buy it because it's a bargain". I don't buy bundles if nothing interests me at all, but 1 game is worth $1.
It's also HumbleBundle, so if I feel I've been a cheapskate by only paying ?0.50 and ripped them off, I can simply just increase my payment amount later.