Yes, you should buy it, rather than steal it! (little joke)
If you already have the Game System why buy First Light? It will feel like buying Spirit Queen's Torment or Prophecy of Telor, because so many of the assets are just like the Game System, but now you don't get any fancy colors for the creatures. Actually those sets could be a nice compliment to this one...
What you get:
New challenging quests (by the OG man himself Stephen Baker), new hero sculpts, dragon... rat tiles could be used for something... cardboard doors could be useful for quests that call for too many... caverns board for extra quest ideas. I guess the bone dice are cool. One suggestion was that the monster standees could be laid flat to represent defeated monsters. The furniture flats could be used as a painting guide for your plastic furniture.
The extra spells are revised, I think they're a bit nerfed but having an extra set of them could be helpful for players who want to have more than one wizard in a game (I know, the "rules don't allow that" well you can do whatever Zargon says at your table, so there!). Or you could have like two of each spell to represent that you get to use each spell twice, maybe for a more advanced magic user?
First Light could be "travel heroquest" for all we care. You can roll the dice in those trays (without removing them from the box lid) and it works well enough.
To support "the cause" of more quality HeroQuest.
The real reason to get it: BOOT TILE.
Don't worry they've confirmed this isn't the beginning of turning existing sets into standees... and it isn't going make the original Game System go away. It's compatible with all existing expansions (and will be with future, both it and GS will be that is).
As Bohemyus pointed out on our last TheRantCastPodcast some people are already playing First Light using the App and their Game System assets already (you can just play the cavern board ones inside the app if you want to, even though that isn't what was intended).