Marsh's issue is that he is likely only a platoon player. But an 887 OPS against RHP going into today is a very good platoon player playable in LF or anywhere. I think Rojas will be back by August unless we trade for somebody like Robert. Dahl is just not that good. He deserved a shot after what he did in AAA, but AAA ain't what it used to be (and Dahl really was platooned there and DH'd most of the time).
The team just won a series against a pretty good team. There is no crying need to solve these issues today. They deserve a month or two to figure things out.
He is an elite defensive LF and a better than very good offensive player against RHP, which is the strong side of the platoon. He's also still the youngest and least experienced starter on the team minus Rojas (or at least, the same as Stott). It is disappointing he hasn't taken that leap forward but no different than what's been happening with Bohm and Stott at various points in the last few years. And because he's a LHB and a good defender he is still more useful than the current Casty (though I'm still hoping) or last year's Bohm, to say nothing of Merrifield and Pache (who are more "bench players" than "platoon players.")
Entirely possible that Dahl loses his spot to Clemens between now and the deadline, or that they really do give Sosa a shot in LF. But he certainly won't field as well as Marsh or hit like an OF either, just possibly better than Merrifield.