I think there's a certain amount of scaremongering going on here. I'm SEARES and would recommend it heartily to anyone who wants an active military role within the RNR, however i fully appreciate that not everyone wants to carry a gun for a living, indeed some are deeply uncomfortable at the idea.
As a heads-up the way things are going seems to be that everyone will now get basic weapons training as part of their Raleigh course, indeed i believe this may have already begun as a basic introduction to a more military way of doing things. My understanding isn't that things are rigid after basic training in terms of branch choice, although as others have said requirements at the time immediately after you pass out will dictate your choices. In this respect nothing has changed for years and is entirely the same as the RN. It stands to reason that you will only have choices that currently have vacancies. (You can't go to a civvy employer and tell them you want to be the managing director and expect to be in that job the next week - not the ideal analogy i know, but it serves it's purpose!).
As a SEARES rating with many years experience i can see that with current deployments it is an absolute must that people be able to competently carry a weapon (we are after all a military organisation). The prime example of this would be logs branch folk who are often deployed on the ground in various hot, sandy places, who while not 'blanket stacking' (joke) may be required to do sentry duty, convoy security etc......
Not sure, but this may not be as true of say comms branch etc. but who knows what the future will bring. We all need to be flexible to a degree and whilst it may not be something we do on a daily basis, we may all from time to time be required to carry a weapon in the course of duty.
As i said before, bear in mind first and foremost that you are joining a military organisation and be mindful of the fact that with that comes responsibility of various kinds.
P.S. I'm not directing this particularly at the originator of this post, but as a general note to anyone thinking of joining.
Good luck and have a great career. It's what you make of it!