Tags have a couple of purposes and the Tag Perk is not useless.
1. Tags specialize what skills you want to use like if you want to use Guns then you'll probably want to tag the gun skill and possibly start out at 7 Agility
2. Tags can also help overcome 1 points in Attribute like you could put Charisma down to one but Tag Barter and Speech. That way you don't have to worry about Barter and Speech being in the single digits but at 23 which is a decent starting number. Also by doing that you can raise an attribute that you really like up higher then you normally could.
3. Tags can also help some skills reach certain numbers that you will need like Lockpick, Science, or Repair
4. The Tag Perk is basically a 4th Tag with the equilivent of a full levels worth of skill points into a single skill.
All valid points, if you feel you have an excess perk slot to use for it. But with only 15 perk slots in the vanilla unmodded game, I have never ever felt I had one to toss away like that. If you are patient and plan properly, the skill points you need will arrive in plenty of time as you level along. Using a valuable perk just to acquire a handful of skillpoints you will get later anyway, is hardly worth losing a more valuable perk that can actually give you real positive benefits which are unavailable as per-level skillup points. In every game, there are only 15 perks you can get, and at least a minimum of 40 perks I would take if I could, and none of them are Tag! By the end of the game, you will be looking around for worthwhile things to try to put skillup points into, and not finding anything all that useful or thrilling. Then you'll think "gee, i sure wish i hadn't wasted a perk on Tag! instead of something actually useful to my char's performance..."