i always say, go for what you're passionate about and have deep conviction of - otherwise it would be easy to succeed and be happy in it.
most jobs seem to be like that. you start by doing the crappy tasks and then you rise up the ladder. have you checked whether this is the situation in your field?
i know a couple of people who've been RA's first and then gone on to do their Phds. so i don't see that as being a problem.
though am probably not the right person to be advising you coz i've never been in your situation, i'e. work or phd, then RA or phd. perhaps someone more experienced can elaborate further.