Same reason I hate the idea of moving Pentecost from Catcher to another position. His value is that of an above average hitting catcher. Move him to another position and his value takes a nose dive.
Yep. I don't see the need either. Martin is 32 right now, meaning he will be 37 by the time his contract ends in 5 years.
Meanwhile Pentecost is 22, has yet to start playing this year and will likely either begin in Vancouver or Lansing (likely finishing in Lansing either way)
Upshot is that you can bring Pentecost steadily and he'll be ready as Martin is winding down.
2015: Martin starts in Tor, Pentecost returns from injury and finishes in Lansing or Dunedin
2016: Martin starts in Tor, Pentecost plays in Dunedin/New Hampshire/maybe a bit with Buffalo
2017: Martin starts in Tor, Pentecost plays in Buffalo and maybe gets a September taste of Toronto as a backup
2018: Martin mostly starts in Tor, Pentecost plays maybe a little in Buffalo, but spends most of the year as Martin's backup
2019: Martin platoons with Pentecost.
2020: Martin's contract is up, leaving him to either sign and be a backup (where I'd be ok with him DHing on occasion because of the value as a backup and mentor), Pentecost can take over as the starter.
By 2019 Martin is 37 and Pentecost is up in the bigs by the time he's 25 and getting a fair # of starts at 26/27.
It's maybe a little older that you might expect for a prospect drafted as early as Pentecost, but making it up to the major leagues in your mid 20s as a catcher when you're not a ridiculous prodigious talent is pretty solid.