Well for a good/better WLAN-Connection you always need two strong ends, so it don't really helps having a nice Alfa if your router only has a 2dbi antenna or whatever. So the improvement is not that great because my accesspoint is a weak and old piece of hardware ...
Same thing if you wan't to use aircrack-ng or whatever, you have to be in range of the accesspoint.
But if you just want some passive wardriving, such a big antenna is a great thing to have. But i haven't compared the 5dbi vs. my 9dbi antenna yet, because here isn't mutch wifi traffic around (I'm living in a smalle village).
I bought the alfa and the 9dbi antenna in a bundle so it wasn't that expensive, but i guess i wouldn't need such a big antenna because most accesspoints / routers don't offer the same power as I'm now doing.