Miguel Olivo notes that it took him some time to develop behind the plate:

Montero is expected to be primarily a DH this season, spelling starting catcher Miguel Olivo behind the plate for 20-40 games. Olivo, for his part, preaches patience, pointing out that he also reached the big leagues in his early 20s and admitting that it took him “2 ½-3 years to really realize what I need to do behind the plate.”

“He’s young and he can hit already. That’s not a problem,” Olivo said. “He needs to get better at receiving and blocking, and communicate more with the pitchers. It takes time, though. I had the same problems. … That’s the thing he needs to go through now. Maybe a couple years, one year and he’ll be ready to do it.”

Jesus Montero will be much more valuable to the Mariners as a catcher who can hit. The counter argument is that his bat is so good, why takes years off it by allowing him to catch? I suppose if Miguel Jesus plays his whole career at DH and winds up a .300/.400/.500 hitter, we can have a great argument about a DH making the Hall of Fame.