It's not necessary in the sense that it 'fixes' anything. The Dud cards from the base game were errata'd but the changes are available on FFG's site, just play them as though they had the text that is suggested. Also make sure to use their new "draw two" rule for the Invader, where you draw two no matter how many Marines there are. The difficulty mods are nice but again those are completely free on FFG's site.
It is a cool expansion to have, though. The new monster types are interesting and have some cool power combinations, relying less on the brute force that many of the base game beasts employ.
The airless terrain adds a nice wrinkle to some of the scenarios.
There is the PvP that is a decent addition. Capture the Flag doesn't work too well but Deathmatch is a great way for you and five friends to get some 3v3 in.
The minis are top notch, the new scenarios are great and better balanced, and there's enough cool stuff that you can pick and choose from to spice up your game. So worth having, yeah, but not totally necessary. Assuming you play with their errata, that is.
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