It’s quite viable if you want to make it work. But, with a druid, it’s a difficult approach. On normal and nightmare thorns auras and effects are brutally effective. However, once you reach the hell difficulty, everything has a 50% resistance to physical damage even for non immunes. Given the high health pools as well, Thorns will eventually kill any physical attacker, but it’s so sub-optimal in terms of time that there are just better strategies.
It works very well with a necromancer because they have the amplify damage curse that lowers physical resistance by 100%. This means the monster’s protection is stripped away and dropped another 50% extra below zero restoring the power of thorns. Unless the monster is immune to physical by default. Also, a necromancer can have 10+ skeletons, 10+ mages, a golem and 1 to 20 revives with this strategy.
This is opposed to the druid’s spirit, 1 bear, 8 wolves, and 1 vine. You could make it work as a supplemental option if you wanted to use a weapon that casts amplify damage on striking. The best options are the lacerator throwing Axe or putting Atma’s scarab into your amulet slot. But, short of that it’s mediocre supplemental damage at best.