I'm just going to add myself to the list of people not understanding why you're wanting to play in drop-a. As others have said, nobody in your audience can tell.
There are good reasons for weird tunings. If it pushes the song into a range where your vocalist can rock it, perfect. But you're doing the exact opposite: pushing the song out of the vocalist's range.
This isn't a case of having a bad vocalist, this is a case of having bad instrumentalists who don't understand how to transpose music.
There are plenty of other reasons for wierd tunings as well. Drop tunings are more common in the metal band scene as they allow you to bar the bottom power chords with extra speed and precision.
However you do have to pay attention to your vocalist and what he/she can do. Our vocalist for Cassandra Syndrome has a 4 and a half octave range so we have a lot of flexibility in our melodies and rhythms. But I would consider that ability more the exception than a rule of any kind so other bands must do what they must.
: Do you guys think we should just skip the drop A and move on to drop G? What are the advantages of drop G?
About the only advantage of Drop G would be that you would be able to find a new band since you won't be heard in the mix. You can look to Animals As Leaders for a reference of that kinda sound, even though they are actually in Drop E (the Eighth string on their guitars are tuned to the same pitch as the E string on a standard-tuned Bass).
Tuning down half a step occurred because in many cases it does sounds a little heavier and "slightly" different to the average listener- Kiss were actually one of the first to do it, as was Van Halen.
But then things REALLY got out of control with the advent of 7-string guitards... and you now have DropA, C#, Drop C, Drop D, and even Drop A# (Breaking Benjamin)
When I was in a cover band and they suggested ANY of these F*KED up tunings, I told them, you can either buy me (3) other basses, keep them maintained and in the correct tuning, AND transport them to every gig, or we either do everything in Drop D (my hipshot) or Standard E. (because I am a 1 bass for the gig guy)
But then you get the "Well it just doesn't sound right, I was listening to it on the way to practice and it just it isn't as heavy without that Low C..." Other solution don't cover it if it isn't in standard , half step down (moved up to standard) or Drop D. Just for giggles I tried playing "Killing in the Name" on a 5-string, and while it can be done, it REALLY loses something from the drop D tuning.
Tuning half a step down is done for a lot of reasons, mostly for the guitars and singers (U2 for example records in E standard, but plays live in Eb Standard to help out Bono's voice. Metallica uses Eb Standard to reduce tension on the larger string gauges they like to use, offering plenty of fundamental and tone while not being hard to play.
Other bands use different tunings because it is the way they want to sound. And especially in metal music where there can be a lot of bouncing off the open string you really do need to be in that tuning. I can play Killing in the Name Of on a 5-string as well, but it's a lot easier to play in Drop D. If I do my job right, the crowd probably won't notice the difference between either one.
BTW, most songs that are played in tunings above B Standard use six-string guitars

. Occasionally you'll have someone like Devin Townsend who plays a 7-string tuned to Open C (which is really wierd) but has a bottom G string that he really doesn't use much, same thing for Mudvayne who are in Drop-C but when they use extended-range instruments (namely Ryan M using a 5-string bass with a bottom F#) those guys just kinda stay away from the bottom string.
Short point - Straying away from E-standard tuning isn't quite the Devil some musicians make it out ot be, but there can be some really important 3rd- and 4th-order effects (like making the band mix well) that need to be considered. In Cassandra Syndrome we play in B Standard, but normally our two guitarists are working in different octaves to spread our sound around the sonic spectrum and keep things clear. We are also very considerate of our EQ's and how they mix with everything else, I'm not sure how many bands do this (and I've heard some where everyone seemed to have theirs set for Obnoxious :/ )
I think that's all I have to say for now. Have a good day
Peace,
Greg