Actually, he is. He has used RBBC in all but 3 years he has been OC or HC. 07 with Willie Parker in Pitt, '10 with Mendenhall in Pitt, and last year with David Johnson
I think you're stretching the definition of RBBC just a bit. Arians likes to use 2 RBs, but the lead back almost always gets 15-20 carries per game:
Cleveland, 2001: James Jackson on pace for 283 carries but got injured
Cleveland, 2002: William Green (243 carries) and Jamel White (106 carries)
Cleveland, 2003: William Green on pace for 324 carries before injury (RBBC after that)
Pittsburgh, 2008: Willie Parker on pace for 305 carries before injury
Pittsburgh, 2009: Rashard Mendenhall (242 carries) and Willie Parker (98 carries)
Pittsburgh, 2011: Mendenhall (228 carries) and Isaac Redman (110 carries)
Indianapolis, 2012: Vick Ballard (211 carries) and Donald Brown (108 carries)
Arizona, 2013: Mendenhall (217 carries) and Ellington (118 carries)
Arizona, 2014: Ellington (201 carries in 12 games), Stepfan Taylor (63 carries), Kerwynn Williams (53 carries in 5 games)
Arizona, 2015: Chris Johnson on pace for 285 carries before injury (David Johnson averaged 18 rushes per game afterwards)
So, 11 out of 13 seasons (including the 3 seasons you mentioned), Arians had a lead back that got at least 15-16 carries per game.
If Williams gets 16 carries per game and averages 4.0 YPC, that's the equivalent of 1024 yards. Which makes him a fantasy starter in most leagues.
Granted, that's not as exciting as winning the fantasy lotto with an unknown 3rd-string RB who rushes for 1500 yards and 20 TDs, but those guys don't come along every week.