We do use exit velocity off the tee as part of our tryout for this simple reason: it doesn't tell me who CAN hit, but it tells me who CAN'T hit.
Like many of you said, a great exit speed doesn't mean that they will be able to hit, it just means that they might be able to.
In the BBCOR era if a varsity player is producing exit speeds in the 70s, they probably won't be able to generate enough power to hit at that level.