I do backup to 2 external disks for Time Machine (in 2 locations), plus I have another drive for archived images. The good thing is that if you want you can always set either program (I think) to backup your images to an external disk right away when they download. Still, becuase I'm working on a laptop I generally try to keep only about the last month's worth of images on there rather than everything.
Speaking of the archiving procedure, I've actually found that both Aperture & Lightroom will let you move portions of your catalog "offline" which still lets you see a preview of the image even without having your external drive plugged in. I always found Lightroom worked better for me with that, but that's because for some reason Aperture 1.5 would never ever finish processing the previews (2 may be more cooperative in that aspect).
Back on the iPhoto topic, I haven't had any hassles using it for just those few things (I think it does a great job at making cards etc!). I just don't use it as my main storage, so I just drag in the pictures I want to use.
