most of the bloggers agree to leave the designers credited link but they disagree to keep the footer of their blog full with sponsored links. i saw themes with decrypted footer but only the sponsored links taken away.
Yes, it is beneficial, one of my sites rank top 10 for a major keyword simply from about 8 sponsorship links on wordpress/html themes. It works for now, but as Google is taking a stand at this 'link buying' phenomenon, I'd expect it to carry less weight day by day. Don't over do it, there's a well known member here (who will remain anonymous, but he'll know I'm laughing) who sponsored anything and everything and received a serp/PR penalty for doing so - or at least a combination of buying sponsored links/buying links/PPP, etc. Keep it under cover & quiet, on 'related' themes and you'll be fine. Ie. Don't sponsor a theme with computery images all over it if your site is about hill walking
I have thought about doing this but I find it a problem unless you have a universal niche that could fit just about any type of site. For example, I found a theme I really liked for my blog. I was willing to leave the links on the theme. However, all the links were for wrestling shoes. For some reason, it affected my adsense ads and that is all that would show up for ads. Since I did not want to be dishonest and remove the links, I just changed themes. That is one of the major problems I see with sponsoring themes. People will either delete your links or not use the theme because of your links. The other one is like others stated, Google may not count these as quality links.
Well to get traffic and some useful backlinks it is benificial but google filters these links very easily