Under some circumstances, yes, you should have a reason: say Google makes it require an effort to keep the accounts separate (I think this might aspect be the case), then you probably should have a reason for thinking its worth the effort.
No. The accounts are already linked! My accounts are not separate. This is a common misconception apparently: my Google accounts are already linked in a single Google Account. I'm not very happy with it, but hey... it's comfortable.
I just refuse to have the social aspects of Google imposed on me through Google+. I don't want my mum to know I liked "Used Panties Exchange" in Google Play. Heck, I don't even want Google+! It's a service I don't need, don't want and feel it's just getting pushed on me.
But well, even then... it's just because I just don't want to! Why don't you use <insert here a service you don't use>? Because you don't want to. Period.
It's like jokes: they're not funny if you try too hard. I might've tried the service by myself, but they just blew away any interest I had. If they're so desperate for users it certainly can't be a good place to be.
