This is an often debated subject, and admittedly, don't do well in searches but i will give you my stance.
Trading off your primary is what is done by cavers and the DIR crowd. You should be able to do this easily and without complications, if not then I suggest more pool time and practice.
If I don't have my BC with me, I likely don't have my regs either, so that isn't a problem for me. I can't see myself diving somewhere where I wouldn't have my own gear with me anyhow.
Since you are constantly using your auto inflate, your muscle memory is instinctual to know where your alternate air source is. I contact my inflator/air source dozens of times over the course of a dive, compare that to someone with a dangling octo dragging in the sand or stuffed into a pocket someplace because it keeps dangling in the sand.
Watching new and experienced divers, those trained to switch out their primary and grab an integrated secondary air source can more quickly do the drill and with less fumbling than even more experienced divers using an octo that they are more likely to fumble for in a panic situation. YMMV
Solutions would be to run a longer hose on your primary if you don't feel you can handle a shorter hose well and I even know a DM buddy of mine that occasionally attaches an octo on his setup also.
An octo really starts to look like an octopus when you have primary, HP, DS inflator, BC inflator and an octo hose coming out of it.
IMHO, its not the streamlining but the instinctual muscle memory recall that makes an OOA situation that much less stressful, when you don't even have to think about it. Of course, with sufficient practice and training, this can even be done with an octo. Just make sure your octo isn't dangling in the sand like most of the divers I see. Look around this board and see how many dives have been distracted or aborted due to leaky octos.
This is one of those topics that shouldn't come up in "polite conversation," sort of like politics, religion, force fins or leisure pro. Thanks for asking though.
