Phaeston is uniquely perfect for Marksman since it has lots of spare ammo and is lightweight enough for the Turian Soldier to be able to comfortably use Prox-mine to debuff and have a low recharge going into Marksman, and all that extra headshot damage makes it into a murder machine against mooks. Not that it fares too poorly against bosses, either -- in a solo it might struggle, yes, but soloing is an absurd sideshow and not the point of the game anyway, so who cares.
I think it's worth singling this part out for emphasis - it really is more than simply a case of everything being good on the TSol.
I find the debate very interesting, as there are people I tend to agree with on many things offering opposing views.
The challenge points I've scored with ARs (as opposed to the ARs I use most, which are two very different things) are in the following order:
Harrier (17), Lancer, Saber, Typhoon, CAR, Striker, Phaeston (8), ADAS, Falcon, GPR, Spitfire, Revenant, PPR, Mattock, Valkyrie, Avenger, Argus (2), Vindicator.
I vowed never to use the Argus again after completing the challenge points the first time. Then a similar discussion to this one convinced me that I was a scrub, so I did the challenge points again, hoping to see what I'd been missing. Then I metaphorically airlocked the goddamn piece of crap. I rarely host, so I can only assume that the required fabled Turian stability was lacking. Or I'm just a scrub. It's ok though, being a scrub sits fine with me. 
I went pugging with Stephen last week, using a TSent / Phaeston, and the game was followed by a one-word message - "lolphaeston!" Trust me, it was meant in a good way, and I guess I should emphasize that I don't care about score, but I do care about extracting. I'll extract if I have a Phaeston, 9 times out of 10. The Fat Turian it will inevitably be in the care of has little to do with it, except adding style and swagger, obviously.