I think it really depends on the control in the clinch. If the attacker with knees is controlling the clinch he can effectively land knees to the body. He can create the space to land it w/out struggling to keep it standing or allowing his opponent to escape. Or if he has good armhooks in he might be able to bend his opponent down to knee him either in the gut or head. Basically if they're still fighting for control in the clinch the most knees I see are weak knees to the legs.
It also helps to consider the fact that catching one leg at the cost of eating a body knee does not neccesarily equal a takedown. Nuts single leg counters aside, the ability to survive on one foot and make your way to the cage wall to work out a response usually makes trying to catch a knee in progress a bad idea when it comes to cost/benefit. As has been said before, while you can hug a straight knee with relative ease assuming you don't get rocked by it, it doesn't put you into an ideal single leg anyway. On the other hand, round knees are most often seen when fighters are belly to belly or at least upright. Here your head is safer and your back is straight so you can much more confidently attempt a leg takedown, whether it's a sweep or shot.
to tell you the truth i have seen a few more round knees recently, espcially in the "ultimate fighter" show, They seem to only score takedowns off them when throw to the head or upper body. If thrown to just above the hip and down, (they throw them to the leg allot) they seem safe. Guess its the same with useing them in self defence. In "Real heads, knees and elbows" geoff thompson says "round knees are effective when thrown to the knee and thigh. Sometime you can attack the body, but be carefull because you might get your leg caught and taken down, and only try for the head if you are VERY well trained."
