Watch the Hammer-ons Overview online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
You have to hammer-on your bass notes because your right hand is already busy with percussion. You have to press down FIRMLY to get them to sound with the same volume as your drums.
I hammer-on about 90% of my bass notes, so there are some times that I'll pluck bass notes with my thumb. It all depends on what the individual riff calls for. You can experiment with this yourself.