Bomber-timing levels are the perennial pet-hate of non-LFers - see any walkthrough or video series of the original games. Definitely want untimed for singleplayer. It might even make fling-bomber puzzles a whole lot nicer
Multiplayer, naturally, is somewhat trickier (but not that much, it turns out):
Timed bombers are powerful but hard to control - that balance is essential. The instantaneous removal of terrain is faster than any digging skill will allow, but in the chaos of multiplayer it probably won't explode where you want it to, even if you're a highly skilled player. Removing the timer gives too much control, and then the other skills can't match its power. With knockback, both aspects are magnified - now you're actively redirecting the other lemmings, rather than just altering the terrain they walk on.
Instant bombers won't work in Lix multiplayer (especially with knockback) unless you limit their count severely, or restrict their assignment to walkers only (which is a huge change and may not even help much in this situation). Trap a bunch of lix in a pit above your goal (even a long way above!), and no one can stop you from blowing a path all the way down. The loss of lemmings through exploding is a small price for the large cluster that you kill or steal.
Once upon a time, I actually suggested L2-style bombers as a means of balancing the power of blockers. (The L2 "Bomber" is the same as an untimed knockback exploder, but without killing the lemming.) Thankfully, a
batter solution was found
. Now the blocker-removing function of bombers hardly matters any more.