make more sense to have more MPs than CID or SWAT. SWAT's a civilian thing. Half the Joes can probably do SWAT stuff any way by virtue of being either Special Forces trained (Flint, Falcon, Duke, Wet Suit, Torpedo and Wet Down the SEALs, et al) or through being Ranger qualified and trained for Close-Quarter Battle (Footloose, Grunt, Crazy Legs, Beach Head, Stalker et al.).
MPs are responsible for detaining prisoners of war, traffic control and all sorts.