Well depending on how much time you have, injection can be useful to speed things up. At the slight risk of increasing your noticeability if someone's actively watching the network, or the access point sends alerts to some sys admin about suspicious traffic.

I've sat at mostly-inactive WEP points to pen test with my N900 - I've occasionally been able to run out of battery before Kismet collected enough packets to crack the key (from a technical stand point, I'm sure that I could narrow the listening to just one channel, instead of channel hopping - that might help get more packets. But the point is, injection can reduce those multiple hours to a lot less. If I understand things correctly. I could be wrong, not my area of expertise.