Welcome to Librarium Online!
Ever since I got into Chaos I've wanted to use a Daemon Prince or Exalted Daemon with Master of Mortals in a mortal army. I kind of feel like its not very viable though, due to the large points sink of the Daemon Prince combined with relatively expensive core and special units. I've noticed that almost nobody on the boards has used this type of leadership. I'm not as worried about fluff, although I don't think a daemon leading a mortal army is especially fluffy either. Finally, if I were to use that option, which god's army would benefit the most?
I have actually used this combination to some success. While not a normal pick for my army I decided to drop on of my exalted champions out to, I suppose you could say, "promote" my usual mortal lord to a daemon prince. I played the list against a bretonnian, orc and skaven army.
Against the skaven the daemon prince faired far worse than I had hoped. I had hoped he could fly around the board causing terror left and right only to have my hopes dashed as 4 jezzails and warp lighting cannon made short work of him on turn 3.
Against Orc I used him more as a magical support and a flanking unit, which he was quite good at. Forcing an orc unit to take a panic check and then a terror check worked out in mavor once or twice. He was sadly killed by the black orc warboss. Outside of the greater demons, most daemons just don;t seem to have much in the way of survivability.
I was surprised that he did the best against bretonnians. I flew him in between the large lance formations and march blocked forcing not only some delicious bottlenecks but the subsequent terror checks collapsed the brett's entire flank. With the Blade of ether and soul hunger he was able to tear through knights and did well on his own as long as he stayed on the flanks or rear.
Overall, I wouldn't include a daemon prince regularly, but they add a great deal of tactical flare to your army and your opponent rarely expects to see one.