I never liked this either, I would rather like it that they filler kills the mob instead and keeps going.
to keep placing blocks while an entity is there might be a problem, as you can't place a block when an entity is in the way, so the filler would actually need to kill the mob instead. then again, people might then use the filler as a method to auto-kill mobs xD
my other "gripes" with buildcraft are the way facades, pipe plugs and gates work on pipes. they're hard to place, and even harder to remove without having to break the actual pipe.
I also don't like how the assembly table and advanced crafting table need lasers. don't get me wrong, I like the lasers, but I wish the machines had an energy buffer so you could charge the tables when they are not working, and get your work done faster when they need to work.
I also don't really see a point in the advanced crafting table, because if you need to create large amounts of an item you might aswell just do it yourself. I also don't like how slow the assembly table is, and that it's not really early game. I would use the gates alot more often if I could start making them alot sooner then it is right now. I also don't like how you can't really automate the assembly tables.
and I miss my builder