well the question was asked..so I take it at face value.
pretty much what everybody else said...I mean everybody who took the question seriously and not the forum clown, village idiot, etc..
but here is how I would honestly answer it:
I am in my 40s with two young kids... the set times make it possible for me to know when I can leave and still make the show...time is a luxury I don't have especially at night with having to get the kids fed and to bed..same goes for most people I know in my age group..... I can't be at the club much before the band goes on..if a band goes on at 9 that is just about the time I can make it.. . my family's schedule doesn't allow much else..I rarely can meet people before shows for drinks or whatever.... also the set times allow me to be somewhat calm about making the show, trying to pick sidehatch up, or whatever...otherwise everything becomes pretty chaotic and messy ...then you get to the club all frazzled...this is precisely not the point of going to see a show for me... I'm trying to get away from the stress not create more of it! if I get there frazzled it will take a few songs to relax..
the other key reason the set times come in so handy for me is I like to pre-party before shows..its been years since that involved drugs unfortunately but I do like to drink a few before the show...then when I get there I keep drinking so i don't think it cuts down on what I drink at the club. .in fact the opposite.. if I show up sober to a show I'm less likely to drink than if I show up with a little buzz...I do have to put in a plug for people that like to get legally high in DC... the set times help them get there for the show feeling on Cloud 9...instead of sitting around with a killer buzz being wasted.
i can't speak to what reasons would spur the 930 to eliminate something that makes its customers very happy, adds to the quality of the experience, etc... it could be simply the idea that people might get there earlier and they might be able to sling more drinks and make more money.. .. or it might be that too many people show up close to when the main act goes on (although I always do this and never notice long lines, etc.) putting a strain on door staff and also meaning too many people opt out of giving the opener a chance..... I'm not sure I entirely buy the money reason... there is a whole hell of a lot of good will that is engendered when the 930 Club treats people like adults and I think it comes back to the 930 Club..
I have always thought that the club posting the set times was really respectful of MY time..when I go to Live Nation events- rarer and rarer- I sometimes think I've been taken hostage and am being offered really crummy beer at exorbitant prices etc.. when I go to the 930 at least I feel like I am allowed to decide when to get there for the main event because i am told when it starts..when what I am interested in starts....and the beer options are good (for me)..going to a 930 show from arlington va, arriving there and walking in, getting my tab at the bar going, drink in hand, and the band i'm there to see going on right then- like clockwork- is AWESOME! makes you feel like a million bucks.. at least that is the way it is for me..
I guess I like it a lot the way it is..but i would adapt and i bet I can estimate when the main act goes on fairly well... i can also reach out to Eddie on the board or the club via twitter or whatever.. but there would always be that nagging insecurity that would come from not being sure...to illustrate... recently i went to charlottesville to see franz ferdinand.. i tweeted to the venue when they thought they would go on cause going down there for me was a huge deal and i can only leave when i can leave...they said "aiming for 945".. i took that as 945 although in the back of my mind i thought "hmm..sounds a bit late".. I ended up getting there at 945.. the band was on like song 5.. that sucked... the Jefferson Theatre is no 930 Club.