Currently Dimension tool requires you to orient your cursor when creating a radius or diameter: inside of the clicked shape to create a diameter or outside to make it a radius.

This is very nifty, sure.
But:

1. The tool hardly tells you about it when you draw. The only hint Ai give us is a blue bar (that we all tired of seeing, because everything in Ai now pops and shouts and distracts — I’m speaking about use, Text To Vector!) — I attach the screenshot of it for those who blinked and missed it.

2. If I work with a tiny circle and I want to mark a diameter for it, I’d have to precisely aim inside of this little area — and this give me very little control over the angle of the line produced. Especially important, since you just ignore Shift to constrain the angle, which is respected by any other construction tool in Ai.

Here is a request about Shift, BTW: http://illustrator.uservoice.com/forums/333657/suggestions/47734580

So, for this request, please introduce a modifier key we can use to toggle radius/diameter as we draw, without moving the cursor in or out.
Opt / Alt will do.