Hmm. Well I believe that is depends on the person. I personally like to customise my profile and journal skins. Also dA introduced this new tool "more like this" I guess it's also a reason for people who likes to browse art a lot to consider getting a premium membership. Even before i volunteered for dA i used to get subscription just to customise my profile. Hope that helped!
Theyre nice, and truthfully, if I had the cash I would take dA up on that two for one offer, but truth is other than Journal skins and other things to make your profile look all pretty-like, theyre not really useful. You can have some fun with the poll option though and the multiple deviation viewing thing, but other than that, theres nothing else.