Hmm... how do I know if I need tendon support? Carolina has on occasion overreached and she has a pretty decent scar on her left front from it.
Over-reaching is more effectively protected against by bell boots. It would be a pretty dextrous horse who over-reached anough to injure themselves in the area a sport boot covers!
Sport boots are designed and marketed as 1. Protection from knocks and scrapes. 2. Tendon support.
I think the whole tendon support thing is hooey. They aren't custom fitted to your horses leg, they aren't tight enough to effectively support similar to a human strapping a knee/wrist. The only reason I ever use them is for polocrosse, as they are compulsory, and also because of the high liklihood of your horse being stepped on/knocked by another.
For everything else? I ride MG, campdrafting, sporting, working classes, and don't use them. Horses have pretty goo natural tendon support, they put their bodies through a lot of strain even in the paddock, and normally come out ok. I think Sport bootsare uncomfortable, they over-heat the tendons...
It's personal preference, but I don't use them unless I have to. I know that I wouldn't like my legs baking inside neoprene on a hot day!