There are no reasons not to use it. It's a very simple install. Also, when developers compile scripts with obse in use, even if no obse functions are used, obse is required to play the mod. I've played a few mods where it never said I had to use obse, but it turns out I did
You should let the author know in that case. The compile = require thing was only added in v18, and I thought it was "if you compile a script that uses OBSE, but no OBSE v18 features, it will still require v18, but if you compile a script that doesn't use OBSE at all, it still won't need OBSE". I could be wrong though. Anyway, the point is that it being a newish thing and a bit misunderstood (at least for me) means that it may be that the author just didn't know, and if you tell them, they can fix that.
On topic though, there's no reason to have OBSE installed if you don't use it. Despite the 'it doesn't do any harm' side of things, if you don't use any mods that require it, there's no point having it. When you do install such a mod, you can then grab the latest version and install that. It's not like you can forget, since a mod should definitely mention the requirement in its readme.
OBSE is a fantastic resource, and a great many brilliant mods rely on it. I couldn't play the game I want without it, for sure. However, contrary to opinions expressed above, it's not a requirement for a satisfactory game. It is perfectly possible to have a game unreliant on OBSE that still works how you want it, and it's perfectly possible to create great mods without requiring it (ie. RGO, last time I checked).