I had the same thoughts as you just over a year ago when planning out my new Humber rib.
At the time, I knew I wanted one for nav lights, and perhaps one aerial, but then came thoughts on things like solas requirements on radar reflectors etc... and other electronic gizmos that could use the frame.
In the end, I went for the double unit (more cost
) which has a bit more strength than the standard single a-frame - I have to say I'm glad I did this, since I now have 2 aerials on the top, as well as a firdell blipper radar reflector as high up as possible.
Also, this a-frame is massive, and has lots of potential for extras like life rings, epirb unit etc... all out of the way of the main deck space, and relatively dry up there too....
If you went for the single a-frame unit, you could get away with a single small antenna on a fixed dsc unit easy enough since a lot of manufacturers combine the two together now (I went for separate icom units, 503, and DS-100). Slightly over the top I know, but I went for the longest aerials I could afford at the time, and knowing some other local ribs have identical ones on board, they seemed like a good investment - they also look pretty good too on each side of the a-frame
Weather here's a bit iffy today so I may only get out for an hr to do some prop tests later on, but if you're around next Saturday morning, you'd be more than welcome to come down and see my setup. I'll pm you some times later on, since I'm restricted by tides on the lock in Milford at the mo.