What defensive forces? They probably have hardly any officers left... hardly any serviceable starships left. They are probably ripe for the picking.
I'm an avid Cardassian fan and I have to say that this is probably the best choice for them at least until they can stand on their own two feet. It's all about biding time.
I've always felt that having a race just join the Federation leads to that race's dilution, the exact opposite of how you expressed it.
It seem to me that the damage to Cardassia during the Dominion War is always greatly over exaggerated. For example, people always say that the Cardassian race was almost wiped out by the Dominion when, in fact, "merely" 800 million were killed on Cardassia Prime. Also, there were lots of Cardassian ships and officers that defected to the Alpha Quadrant forces during the final battle.
Now, what it seems to me that Cryptic is doing is they are drawing parallels between Imperial Japan and the Cardassian Union as ruled by the Central Command. Both were highly militaristic societies, basically ruled by their militaries, both lost their respective wars, both had to accept the dissolution of their militaries and defence from those who defeated them.
This doesn't mean the Cardassians would become part of the Federation, but close allies who depend on the Federation for defence, much like Japan now is to the United States.
But, I hope the Cardassian Union rises again, and the Cardassians do get their military back. Maybe this can be caused by the Federation asking the Cardassians to commit men and ships to a war with the Klingons, or some other power, much the same way Japan is getting to be more militaristic again because of American pressure for them to help out in Iraq.