Aegis, Yay or Nay?

Im a player who plays 95% tanks, so i often have an opportunity to buy aegis of the legion (whether in locket or Banner form, or just plain). HOWEVER recently people have been telling me im dumb for buying it and that the stats are not worth it.

I buy it every game someone else is not buying it and my logic follows as this.

1. If the entire enemy team is AD except for a tank, i buy it because they wont buy any penetration which will cut down on statik shiv hits, random magic damage from abilities and anything a tank can throw out (tanks are hit the worst IMO when facing resists since they never buy enough pen to counteract it). In this situation ill often get no other MR items since there wont be any crazy Magic damage coming my way.
2. If the entire enemy team is AP, I HAVE to buy it since otherwise squishes are practically dead-meat, so it seems worthwhile.
3. If theres an AP assassin, so I can keep carries just a bit more alive.
4. Theres AP poke so we can siege towers.
5. ETC...

Basiclly I often buy it because I cant see a situation (unless it was like talon, Draven, Janna, Yi, Darius) where it doesnt actually do a very good job of mitigating damage for characters who dont often build MR into their builds.

Is aegis worth it for its stats? are there any situations (besides one where NO magic damage is coming across) where you wont buy it?

Just wanted to here your thoughts

PS im not rushing it every game (except if im vs all Magic), but I often will get it as my MR item

Thanks so much

-macbeth