I always thought it was strange how limiting they were as well. I only have BLM leveled high enough to do trials at all and it did take me a while to notice that the staff trials, which I went to first because they made sense, were the only ones I had any access to at all. Really there are only fringe reasons to ever need to melee as a BLM as it is, but in lowman situations I've pulled out a staff to get a blue proc here and there, so it would be useful to have a decent club (more blue, or red if mysteriously /WHM) and possibly dagger (minimal blue usage, but red procs when /RDM). BLM tends to be a poor example of this, but there are more compelling arguments with other jobs that actually use weapons for which they can't do magian trials. I'm just not perfectly sure what the rationale in limiting the weapon selection was to begin with.
Personally I could do magian trials for a couple months nonstop before I finished everything I think I should have so the issue is rather moot for me, but I'm sure some people really could use some of these weapons that they can't get for no good discernible reason.