To explain the above a bit:
So the game has Horizons DLC but certain players can't upgrade (Mac) or don't want to. They still need to play and since the galaxy simulation etc is interconnected and also because it's a good thing to do they still have to be able to meet with their friends etc that did get the dlc.
As a result each DLC update (2.0/2.1/2.2) comes paired with a non-dlc update (1.5/1.6/1.7 etc). Since so much changes a lot of the changes are applied to the base game as well as the DLC. Passenger missions, UI changes, mission improvements, bugfixes, AI improvements, balance changes etc. The headlines usually don't carry across like ship launched fighters and planet landing/engineers etc but a surprising amount of the updates do.
Personal view, try it again, watch a few vids on horizons. If you like the time in game and like what you see in the vids then get the DLC, maybe wait for a sale. If you don't it won't make the game change dramatically to what the base game is. There's tons more content and things to do with Horizons but the feel and character of the game is the same so if you don't enjoy the base game flying spaceships around then you probably don't want to spend another £20 or whatever currency to find out it is more of a similar nature.