Use the monster locker to unlock each type of monster (High health, looters, and just wonderers.)
Even weak monsters can be useful as they can be used as bait, require little housing and are created very fast.
A fully upgraded Pokey with plus 4 level 6 Monster Housings gives a total of 308 Pokeys. This is great for taking down bases after a tower rush is completed. Pokeys, in a group, are very strong, just like Bolts are in a group. If you drop 100 on 3 sides, you'll have an advantage over the bases defense system. Many peoples yards fall to Pokeys once towers are downed. A strong defense is needed no matter what. Being defeated by Pokeys is the ultimate humiliation.