Dave,
I'm not trying to pick on you. I'm just going by what other "experts" have recommended and what I installed in my boat due to those recommendations. A flanged seacock has a flange that is bolted through the backing plate and sometimes the hull as well to retain the seacock in event of an impact from outside or something heavy falling on it from inside.
What you have installed is a ball valve on a thru-hull fitting. One of the problems with a ball valve screwing onto a thru-hull is that the threads are different. The thru-hull has NPS threads and the ball valve is NPT. This often results in not enough thread contact between the two.
Now thats not to say you don't see plenty of ball valve/thru-hulls used as well as the lingering gate valves you mentioned. The full flanged seacock is supposed to be able to withstand a forceful kick or IIRC a 200lb side load without breaking? The thru-hull fitting is likely to snap off under that load in your installation. Your installation looks pretty open and likely to have things sliding around in that storage location?
Its obviously your choice to use whatever you want in your boat. I would hate to see something bad happen to your boat if that setup failed and I had not said anything. Again, I am merely repeating what I have read in many different publications, most notably Nigel Calders books. Attached is a pic of my seacock install under the galley sink. Hope this clears this up.
Check out this link to flanged seacocks, ball valves and the differences:
http://www.jamestowndistributors.com/userportal/siteMap.do?action=map2&catId=164
RT