Originally Posted by
Plainsman
Another thing to consider is your breathing. Aero is critical, but you don't want to pull them so close together that it forces your shoulders so far in that you compress your chest and affect your breathing. You want your arms to be as close together and comfortable as possible while still allowing your chest to be fully open so you can breathe at maximum efficiency. I think it is possible to be extremely aero and yet sacrifice time/energy output in the long haul. Just my 2 cents (disclaimer: I am by no means a professional bike fitter).
I agree with Plainsman. Too close together and my chest feels restricted. Not to mention my steering seems too nervous. Width gives you balance and balance gives me confidence - which means I'l go faster.
Personal preference at the end of the day though. Be interesting to walk around on a time trial day at a pro-tour event ( or even major tri race) and see what everyones formula is. I'd bet there's a lot of variation.