The original poster was asking specifically about using MTB shifters with road derailleurs. Why did Shimano release a flat bar groupset? Who knows? Marketing?

I once had a bike which had MTB shifters, and the derailleurs were Sora rear, and a generic FD. Never had a problem with shifting. The shifters control the amount of travel per click on the derailleur. So long as the deraileur in question can traverse the range required to shift all gears, you can use the combination.

I have used bar end indexed shifters (designed for road) to shift a LX rear derailleur as well. No problems. It is more important that the FD matches the type of crankset you have on your bike. MTB FD are designed to match the curvature of the 44/46 tooth chainring, while the road ones are designed for the 53 teeth ones. Having said that I have successfully used a 105 triple FD with my 26/36/48 chainset.

I have also used a brifter to shift a MTB Deore LX FD as well .... no problem. I am currently using a Campag brifter to shift a Deore XT rear derailleur.

The key is in the setup.