Same here. Currently have two 27" Dells connected to my Early 2013 MBPR. I don't have a major problem with the builtin keyboard. I am less satisfied with the A1243 I'm using on my 2009 MP to type this, but I spend far less time using it.
One issue I have with an external keyboard is that many of them are absurdly tall, which would require me to use the awkward keyboard slider on my desk. Another is that I've yet to find one that don't hate. Tried a Logitech ergo and an MSFT Sculpt. Would like to try a Gold Touch, but they seem to be available for a touch test exactly nowhere, and I'm not going to drop that kind of cash without trying it out first.
My employer is pressuring me for an early refresh of my 2013 MBPR so they can lease-return by EOY. Trying to figure out if I want to act now and get a 2015 MBPR, or try to hang on long enough to get a 2016 MBP. Puzzled that there are no benchmarks at all yet for the 15". The touchbar is cute, but not something I'd use a bunch.
Another reason I'm using both my builtin LCD and external is WebEx sharing. I don't necessarily want to share IM's, mail notifications, etc. with co-workers, so I place them on the external.