In Wildcard's defense, he did point to Mountcastle's less than ideal numbers last year vs righties as well. Now obviously this year's numbers are a small sample size, but with Kjerstad basically benched, the idea of monitoring Mountcastle to see if his struggles against right-handers continues was worthy of discussion in my opinion.
Now maybe the title of his thread here is the issue more than the conversation. Obviously Mounty crushed that homer last night off Harvey so now some people will be like, "See he can hit right-handers just fine," but at the end of the day, keep an eye on how a player is performing when another may be able to do better in his place is a worthy discussion in my opinion.
Hays had a small sample size this year before he was basically replaced by Colton Cowser before he got hurt. Hays also struggled the second half of last year. So sometimes a "small sample size" might be enough to make some adjustments to a player's playign time.
Now saying all that, Mountcastle wasn't struggling that badly and as @Frobby point out was actually 104 +rc and his defense is clearly superior to O'Hearn at first base.
With all that said, Mountcastle is going to still get the vast majority of at bats at 1B this year against right-handers unless he suddenly drops into the mid .600 OPS wise.