There is a fair overhead to create one, but I do not think this will be an issue in 99% of all the SunPy code. I am willing to be proved wrong, but I don't think we should let that fear guide the design of this document, beyond providing scope for public API functions that don't accept quantity if and only if there is a strong reason to do it.

I realise I have made my point repeatedly at this point so I am going to shut up for a while.