I've never asked an expert in the field what the answer is, so I can't say with certainty. But, I haven't really found a difference. There are those that say the neoprene used in dive suits is different than that of surf suits, so you shouldn't use a surf suit to dive because it won't work well. Maybe they're right?!?! Here's what I know. . . I use surf wetsuits exclusively and have yet to have a problem. I use a Body Glove Method 3/2 (which Body Glove advertises as both a surf&dive wetsuit) for my warmer water dives (about 100 dives in it) and a O'Neill Mutant 5/4 when I dive in SoCal in water down to 60 deg. F. (a dozen or so dives). I dive these wetsuits because they're what I have (I've been a surfer a lot longer than I've been a diver), they fit well, and they keep me warm. I dove a Henderson 3mm and it felt no different and I was no warmer than in my Body Glove. It was just a heck of a lot more expensive. $99.00 vs. $250.00. Maybe the Henderson would last longer, I don't know.
So, to answer your question, I really don't know what the advantages of buying a scuba wetsuit are. If you can get a better deal on a surf wetsuit and it fits properly, I say go for it.
BTW: I've never seen a surf wetsuit without knee pads. Which one are you talking about?