I've tested surviving direct attacks, and couldn't get it to work. I wouldn't be surprised if the OHKO radius attacks are just completely unaffected by Bomb Defence Up.
Anyway, I find Bomb Defence to be a pretty good ability overall. It lets you turn a lot of attacks into not-quite kills. The more you have, the easier it is to survive things like Inkjets, Tenta Missiles, stray Bomb explosions and the like. Remember that if you're getting hit by specials or bombs there's a good chance you're taking small amounts of damage from standing in enemy ink, so any extra subs/mains do help, beyond just one main 4 subs.
At the very least I recommend having 1-3 subs, to boost survival against things like Inkjet near-misses and generally have a little extra HP after bombs explode nearby. 3 subs is an 11.0% reduction according to the formula for Bomb Defence I calculated, which isn't specifically enough to do anything special I can think of directly, but is fairly easy to include in a gear set and helps survive special + sub weapon attacks sometimes.