Look up the specifications for MacGregor 21 on Sailboatdata and you will find a lot of info about your boat that is useful. As the specifications show a masthead rig, like you describe, I would guess that the 2nd forestay was somebodies idea of an improvement for your boat! I think it would make life easier when you are tacking if you simply removed that stay.
It might have been useful to somebody, but you will probably like to keep it simpler. OTOH, if you have a sail with the right luff length for the shorter stay, maybe give it a try, but my guess would be that the boat wouldn't be balanced quite correctly. If the boat wasn't designed as a cutter rig, adding the stay might not be the best idea. I don't think it would hurt to remove it.
http://sailboatdata.com/viewrecord.asp?class_id=138
Actually, this is my boat:
http://sailboatdata.com/viewrecord.asp?class_id=137
Mine is a 1973 and it does show it as a cutter rig and shows the two forestays. I note that in the picture attached to the specs, they are using a jib on the more aft forestay. Perhaps I should give that a try. I actually had not noticed that before.
The diagram also shows a spinnaker. I think I'll wait a while before considering that leap.
Still, in lighter winds (which is what I have been generally experiencing here in Arizona) shouldn't I stay with the larger genoa? Although I like the idea of having an easier time tacking/gybing without having to work the sail past the aft forestay.
More food for thought!