I've been running a desktop/laptop setup for years and there's really no downside for me at this point.
I store most documents in Dropbox (and some in iCloud) so I can access files from either computer. I've started storing photos from 2014 and on in Photos for Mac, so I can edit and organize my current photo library from either computer. And since I'm using Apple Music now, I keep very little music on the laptop.
All archived older photos, documents and video are stored or or connected to the iMac.
I think the rMB is a perfect compliment to the iMac. I rarely use it plugged in, and only connect an SD reader when I want to upload photos from a camera. I really have no need to connect anything else to it with the iMac available. As I mentioned in a previous thread, I bought the base model at a heavy distant from Best Buy last year just to see how it would be "downgrading" from a 13" rMBP. I like it. My plan was to upgrade this year to a fully maxed rMB (and either sell or pass long the current rMB), but I'm going to wait until I see what the rumored revised rMBPs look like before committing. I generally hold on to laptops for a few years – if not longer, apart from this most recent purchase – so I'm willing to wait a few more weeks to see what's in store.
Or I'll just pull the trigger on the m7 ;-)