I have to say, my view was rather jaundiced, beause I have known people in the adscence on post nominal in the form a degree or chartered status have 'bought' a MRAES as a post nomininal, to give the impression they are more qualified than they are when seeking consultancy work. I think it would be better if the M was removed it give the impression of a masters, why not simply use RAES as a post nominal.
You cant buy MRAeS, you have to earn it (then you pay the fees!).
I have come across somebody using "ARAeS", who turned out to be an affiliate. That you can buy, and most definitely gives no right to use postnominals. Very firm words were had from various quarters after I shopped him (those weren't the only postnominals he hadn't earned but was using!).
Why not just "RAeS" ? Well because it's not the correct form, and because there are AMRAeS, MRAeS, CRAeS, FRAeS - and they all imply different levels of professional status.
To the best of my knowledge whilst some employers prize RAeS / CEng, etc. most do not - portsharbour is correct. That said, there are places where it has value. You can only be an E-conditions signatory at the moment if you are CEng MRAeS or CEng FRAeS, and I've certainly found that certain doors do open if you're an association Fellow, but that may not be very evident from outside.
The other advantage to these qualifications, if you are jobhunting, is that it means that you've been shown to meet a certain level of skill in your profession by an independent panel. There is something to be said for having that ability to demonstrate this, without having to go to your current employer for it.