but is it a must have! purchases?
If you ask me, yes. There are a lot of awesome changes it brings. Especially if you like mods, it opened many new doors for mods (changing graphical appearance of ships/buildings through research/upgrades, weapons being able to shoot at more than one target at once, frigates with fighter physics, mod stacking, etc). Much of these things are due to the way Starbases were implemented (like the number of targets and appearance changes), and through modding can be expanded to pretty much anything else. Of course there are AI improvements, like sieges trying to go around your cluster of turrets (as only the most obvious)! 
