I just looked up the catcher DWAR's in 2015, and Swihart was a -.4. Leon was a +.7.
Swihart is unquestionably a good athlete and all around baseball player, but catcher is very much a specialty position requiring unique skills that take time to develop. On top of that, catchers have shorter careers, which is why Bryce Harper was converted from catcher to outfield the instant the Nationals picked him in the draft.
Does anyone besides me remember Doug Mirabelli? He caught Tim Wakefield exclusively and was traded away or let go only to be brought back in a very dramatic way. I say again, catching is a specialized skill, and apparently the Sox don't see Swihart as having the requisite skills for full time duty unless or until, as happened in 2015, other guys go on the DL for extended periods.
So I think tonight is an opportunity to again show Swihart's versatility--he can play the outfield, he can play the infield corners, he can catch, he can do it all--while also getting him some more at bats because in the end I think it is his bat which will secure him an MLB career.