Well, this team is trying to win games as much if not more than it is trying to develop one prospect. And when you look at Marsh, it's hard not conclude that it was right to control his exposure for three seasons, rather than "if he'd just started facing LHP in 2023 he'd have gotten this good sooner!"
The problem of course is they still don't have anyone good to play the OF on days Crawford sits. And the fact that that's really also the only reason Marsh started playing every day does suggest maybe he should have gotten more chances sooner (though it's not like he didn't get his chances).
I reckon there is a baseline level of awfulness that is both bad for the team and bad for the player (who will get frustrated or develop bad habits from repeated failure), and Crawford is well below that line (.393 OPS against LHP. Marsh's floor was .550). Especially since he's not playing good CF either.
Different if he was a RHB. A RH platoon player is a bench player, A LH platoon player is still the starter.