Finks are used at lower levels because it is more powerful compared to the towers. One major problem with finks is that it's housing is not good compared to the number of Booby Traps, so finks are not the best against a base with them.
At a higher level, finks are rarely used. The reason behind this is that it can only take 2-3 hits from a Cannon tower of an average level, and can only take 1 hit from a Sniper tower. For Housing space, you might as well use Pokeys . Use them only against farm bases.
Finks are rarely used in the Monster Bunker because of their low health.