I think that the combo brush should automatically be on whenever you're editing in 'combo allias' mode (press 'O' 3 times) Maybe a seperate option for this in the misc-->options menu. I like the combo brush on for this because of having to place a large object (many combos) at once. However, i hate always having the combo brush on for regular editing because it seems to make the mouse move real slowly.
Does this make any sense?