You can, and I have.
I've had my 180gal up for quite awhile using only 250watt Iwasaki 6500k lamps. I had used bluer lamps in the past so I didn't worry about supplemental lighting. I've gotten good growth, if not the best color display from the lamps that I currently use.
I've just gotten a PC setup and will be supplementing the MH lamps with PC actinics, mostly for aesthetic reasons.
In other words, corals can still grow under MH only, but depending on the lamps that you use, you may not be satisfied with the way the tank looks.