As many of you have seen with my posts about it, my Alcoa bass came to me with a hinged trap door, put in place long before I owned it. The bass needed top repairs and being aluminum and welded together from the factory, a trap door was realistically the best way to make the repairs. At the time of these repairs the bass back was also treated with acoustic paint to reduce the metallic tone. But anyway, I didn't have any decision making related to the presence of the trap door, with the exception being whether or not to purchase the bass for myself.
As some have mentioned, there is a possibility of rattle with a trap door... Mine having 2 hinges riveted onto it means there are moving parts that can become loose. I've been lucky enough to find ways to prevent rattle there, but this being a metal bass with temperature expansion and contraction, things can become loose. I have considered and probably will eventually replace the rivets holding the hinges in place with tension bolt and lock nut assemblies, but for now I have eliminated rattle at the hinges with heavy duty velcro and I'm satisfied with that for the time being.
Regarding rattles and access doors though, our own James Condino has described and shown some rare-earth magnetic clasps that hold his access panels closed with no rattle and no moving parts... That seems to me to be a very good idea.
As an owner of a bass with a trap door, I've found it very useful for a few things, and if I were ever to have
a new wooden bass commissioned I'd probably request a trap door be installed...
As has been discussed, the sound post is a primary reason. For me, having access to the sound post has provided a way for me personally to replace and subsequently to tune my adjustable sound post whenever I want to, rather than having to take it in and describe what I want to someone else. This is not at all to denigrate the value of having a luthier examine a bass, but for me it has been really handy to be able to do this kind of tweaking on the fly and without tools.
Another thing that is handy with a 7"x7" trap door in the driver side C is that it can be left open, providing a sizable sound hole pointing basically at the player. I've attached heavy duty velcro to points where I can keep the trap door open while playing for this purpose. I (and others) can actually hear the difference in both tone and volume, so I'd guess we're talking about at least a 10% difference with the door open. All this said though, if I were going to put a trap door in a bass exclusively to use as a driver-side sound hole, I'd place it in the upper driver-side bout, nearer to the player's ears.
Other than that, I've had some fun with leaving the trap door open for my cats to play inside... They love it. I don't know if I would do that with a more standard wood bass, but mine is pretty battle-worthy as it is setup inside and the cats can't really do any damage there. And it's fun to take pictures of them playing inside.
Of course there is the theatrical side of a trap door in a bass... I've heard of some players keeping their drinks or other things inside. That isn't probably something I'll ever end up doing though.
That said, when looking inside my bass there isn't really a lot of other interesting stuff besides air and dust... Although I did once find a loose washer rattling inside, it was nice to be able to easily remove that. The bass bar is actually part of the top, and it probably isn't moving for at least another century. I can see the neck block and the neck attachment welds which need no maintenance... And I can see the end block and endpin plug from inside; I may replace both the endpin and plug one day, but I'll do that work from the outside of the bass. Other than that, there are a few manufacturer markings in a few areas which are interesting to see, but certainly not worth installing a trap door just for those.
Regarding a fine antique wooden bass, I don't know if I'd have a trap door installed. Even if done expertly and tastefully, provenance is still a possible issue. If it's a nice old bass that I'll never sell, perhaps I would consider it more, but I'd have to not be worried at all about resale value. On the other hand, a trap door could sure make top and rib repairs easier on an older carved solid wood bass.
