I think the three distinct ratings are useful. While a single rating is fine for individual posts, it does not help much in determining what kind of poster the user is as a whole, as it is here:
[quote=Mahz][img]https://dl.dropboxusercontent.com/spa/quq37nq1583x0lf/stzv2sf6.png[/img][/quote]
If there was only one rating, we would not be able to distinguish between users who write lots of helpful posts to users who write lots of funny posts, or if they do a mix of both. These three ratings, when viewed from the user profile, provide a suitably diverse rating of the user, and does not blur the reasons they were up-rated (quality, helpfulness and humour being the three main categories here).