Doesn't harm the game in the least
However, it adds a lot of feature and correct many bugs with the main game. I suppose I could draw a list of bugs it corrects, but honestly I don't recall what it was exactly before the expansion, I know it was different...and I remember been happy with many changes...yet it has been quite sometimes now...so I can't pinpoint all the changes. Pinpointing only 1 or 2 changes wouldn't be fair.
The Viking campaign in itself is interesting. But I didn't replay it much.
Yet...this expansion only improve the game, thereby it is safe to buy it. At worst, you will not know what it does exactly but it will improve slightly your game experience if you don't play in the Viking setting.
The official reviews doesn't pinpoints the corrected bugs, but they are there. Now maybe a patch will address those changes as well, maybe not, maybe it will be poorly coded...I don't know. I know that with the expansion, everything is smooth and fine.
All in all, unless you are hurting for each box you can save, I would recommend getting it with the game, it won't change much, but it will correct quite a few things. Sorry if my memory fails me and I can't pinpoint out what...been playing for too long with them.