This really is just sorta sadistic. If your opponent's got starfighters and you have a limited number of command available for squadrons, these two take the cake for efficiency.
Basically, Soontir's ability makes any engaged enemy that fires at someone other than him take 1 immediate damage. The TIE Advanced has the escort ability, forcing enemy ships to fire at the TIE Advanced instead of any other friendly ships the enemy is engaged with. For 30 points combined, these two magnify each other's capabilities very well. The Advanced can tank a good few hits before going down(especially if you're willing to toss in an extra 9 points to take Vader and his two Brace defensive tokens).
I fielded these two and Howlrunner tonight in a friendly 3v3 game. The trio smashed through six enemy squadrons, with the Advanced and Howlrunner both alive at the end of the game (Soontir, sadly, got placed out of protective range of the Advanced by a ship overlap, and got pounced on by some X-Wings :<).
I'm planning on running these two together alongside a pair of Raiders once those come out. I'll have Tarkin dropping free squadron command tokens for my Raiders each turn, which will let this group really ROFL-stomp around enemy fighters.
A bit tired right now, sorry if there are any shortcomings in my post. xP