There is absolutely nothing wrong with a AMHR registered only colt.
Here at our farm we only use stallions that are double registered though in both AMHR and AMHA.

OH!
The reason is fairly simple, we show both AMHA & AMHR shows and enjoy both.
Additionally the double registered horse has a broader market when it comes to breeding/production time.
Some folks show for whatever reason only one or the other type shows. And we do not want to limit where our horses can be used.
Hope that helps and the above is only our approach things, which seems to work for us.
Good Luck with your decision!!!