Well - NASA's committed to not sending any orbiter without Electra UHF radios for relay....so even though MAVEN's orbit isn't great for relay - it still has one (and it performs well). That said - a dedicated relay orbiter fleet is clearly a must-have before humans head that way.
The problem with Cubesats for relay is power and downlink capacity. The MarCO cubesats top out at 8kbps - and can only do a few hours of downlink per day (using the rest of the time to recharge). The power situation would be worse in Mars orbit due to occultation/shadowing etc. A modest MRO UHF pass with Curiosity might be 250 megabits - and that would take almost 9 hours to downlink via a MarCO like spacecraft. Even an order of magnitude improvement over what MarCO can do would be significantly worse than Mars Odyssey is capable of. I think MarCO is generating around 15W at 1.5 AU. When Earth and Mars are at their most distant, they'll probably be down to 1kbps or worse for downlink.
That said - there's huge scope for spacecraft like MarCo to have a use out beyond LEO. Exploring Phobos and Deimos, for example.
We've been slowly pulling down the best of the images MarCO B took during its flyby - each little 752 x 480 image takes around 10 minutes. It was more like 20 before we asked the gumstix camera board to start GZipping them before downlink