How valuable are research assistant positions to an academic CV in the humanities?
I'm a faculty member in a Canadian university who hires a lot of RAs (and who has sat on hiring committees), so I can speak to this.
In general, you need to really watch the balance you take on. The extra money is great, of course, and if you need the funds you ultimately should take these positions - the hourly rate is good.
In terms of CV building: some RA work is great, especially if it can dovetail with your research project (i.e. accumulating knowledge for your field) and if you can get a publication as a co-author. Others may involve more clerical work. The important thing is that the RA position in and of itself is not a real addition to your CV when it comes to hiring, but the effects of it can be: a publication, skills, or having a faculty member who can write a positive letter of reference.
The reputation or prestige of the faculty member would I think only really matter if they wrote you a letter of reference or co-authored with you.