1) ....no, lots of them can, you're just wrong here. Think about the number of Planeswalkers printed and how many of them interact with your opponent's creatures on a plus - Kiora 1, Ajani Vengeant, Gideon Jura, Liliana, the Last Hope, Jace, Telepath Unbound, etc. Not to mention that even more of them interact on a minus and usually those do so in a way that gives you some kind of tempo advantage at the very least and at best kill a guy permanently.
2) It's an EDH card. Locking down one guy (who comes back with its ETB) at the cost of 2 life every turn isn't good and you're only doing this if you're losing and have absolutely no answer to your opponent's dangerous creature. It's probably the worst possible answer outside of chump blocking and lots of the time its probably worse than just using Elspeth's + to actually chump block. Using her 0 on another creature is probably worse than just blinking herself most of the time since she'll always come back with 5 counters (and thus she never dies since she never can get attacked), but in that case you're not actually doing anything with her but losing life.
3) If it It matches up well with Sorin 2 and 3 its because its third ability is Syphon Mind and her third turn will always reset her to 5 counters without dying. That's why its a better card than what we get in Standard, not because of its ability to lock down a dude for 2 life a turn.
I think it would be an interesting card in Standard, but Syphon Mind on a stick for 4 mana is probably beyond what we normally get in Standard.
A) There are almost 80 PW cards in existence. You've listed almost all of the ones that interact with opponent creatures while going up in loyalty (I could add Tamiyo 1.0 and Sorin 1.0). That is about 10-15% of all PWs. So yeah, "few" is accurate. Talking about negative loyalty abilities is pointless. Of course if this ability was a negative loyalty ability it would be evaluated differently.
B) I mean I could name a bunch of spells that are better than dealing with creatures than PW abilities. The point is that its flexible, repeatable and doesn't cost mana to use on consecutive turns. It's a more flexible ability than your average PW positive or neutral loyalty ability. In formats where you can find lots of unique interactions (old formats, Cube, etc.) flexibility can't be underestimated. "If you're doing that on enemy dudes you're in trouble", yeah you probably are. But you have that option which is what makes that good.