Super Tag Battle is garbage. (This is coming from a guy who owns and has played the heck out of the AES version.) The reason it had such poor distribution and is so "rare" is because it's crap.
The combo system in Super Tag Battle is really, really simplistic (weak x 2, strong x 2, special move/DM). Technically, there are "air combos" similar to the Marvel games, but they're impossible unless you catch the other guy on the upswing of the jump (while both of you are still rising in the air), otherwise, nothing connects. The tag system is also really boring, there's a lot less strategy to it than you might think (you have to be in the tag zone to switch, but the game has SS3 side-switching, so it's actually really easy to make the tag if you need it).
The game reuses (what were then 3-year-old) sprites from Savage Reign. Savage Reign was an awesome game and very well ahead of its time, but it went for huge, coarse sprites with very little detail and animation (obvious contrast to MOTW or LB sprites, which are a lot smaller but look much more detailed and have loads of animation). That worked for 1993, it did not work for 1996. The whole game's animation is stilted (hit "explosions" are pasted on, like the sword slashes in SS4). The way they reused animations was dirty. (Several Savage Reign win poses became "new special moves" in Super Tag.) The two new characters, Sue Il and Rosa, are also very poorly animated and not even that strong or interesting. Everyone else, even King Lion/Leo, uses exactly the same sprites, animation, and voice samples as Savage Reign (except the final boss Jyazu, who is a headswap of Gozu and Mezu, and is voiced by the same actor as Sokaku/Gato). Time wasn't kind to the Savage Reign art, especially for characters like Eagle, Chung, and Gordon. The same kind of low-rent, recycled, poorly animated look and feel of Super Tag Battle paved the way for SS4, SS5, and SS5s.
The one thing you might possibly say in favor of the game is that the artists at least tried to make the background stages look somewhat realistic. This is around the time (AOF3, Super Tag Battle) when SNK's art team was turning the corner from cartoony FF2 / AOF 2-style backgrounds to the more "realistic" ones you see in Last Blade and the later KOFs. If you've never played Savage Reign, you might at least get a kick out of how crazy and over the top the characters are (especially guys like Chung and Joker), but any and all of that crazy was copy-pasted directly from Savage Reign, where it's put to much better use.
If you can get Super Tag Battle as a very cheap MVS cart, maybe try for it. The AES version should still command a pretty hefty price ($400-500 minimum) and is in no way worth it if you enjoy having games that are worth playing.
If you're looking for a good, "old school" SNK fighting game that isn't KOF or Fatal Fury, just get Savage Reign. That game was actually really cool and had some innovative ideas, like how every character has at least one projectile attack (and by the same token, everyone has a projectile reflect ability). Also has a surprisingly solid combo system and moves nice and fast. Much, much better game overall than Super Tag Battle in every single respect. Outstanding English announcer too, as good as or better than Kuroko in SS1.
