Low level tank/healer don't really teach you anything meaningful about how the classes play at high level anyway. Hell, for healers it actually installs bad habits (with the kits as they are now) that you need to *unlearn* to play well at higher levels. Maybe it's different for DPS which haven't regularly had their abilities removed and recycled into higher levels, but I can't think of a single reason to start any class lower than 50 these days, there just aren't enough abilities in the lower levels now. Progression's a lot different than it was back in ARR.