I'm not thrilled with Spencer, but I think the platoon should do okay. I estimate that they should be able to platoon to around an .840 OPS (unadjusted for Shea), which would rank them around league average for RF (10th out of 21 RFs last year). And for that league average production, they're only paying about $1.3 million, which is very little, and allows them the financial flexibility to upgrade if they're in a playoff race.
I think platooning in general is under-utilized. There are a lot of situations where 2 cheap, below average players can perform better than a more expensive regular.

By the way, the math I used to get to .840 was approximating their OPS based on the last 2 years, and multiplying by the typical platoon advantage (which is 1.17 for LHBs and 1.08 for RHBs). That came out to .854, but I arbitrarily downgraded it a bit, figuring that there would be times when the platoon didn't work out because of multiple relievers.

If you looked at their actual platoon splits the last 2 years, they'd wind up around .810, which is still around average.

So, I think in conclusion, I'll estimate somewhere in the .800-.850 range.