If Merrifield isn't better than Josh Harrison he'll also be worse than Cave. But that's not really the point of comparison in any case. For one thing Cave hasn't necessarily lost his spot. For the other, with $8 million the bar shouldn't be "better than Cave" but rather, "better than Merrifield."
In both cases, the Phillies are clearly settling for a lesser player (and in Merrifield's case overpaying) because they don't have enough playing time to attract a better player. Inasmuch as that stems from a commitment to Rojas and Marsh, I can respect that.
I don't think it really matters who loses their 40-man spot to Merrifield though. It never made sense to carry both Pache and Rojas so Pache was getting traded or DFA'ed anyway. And if Merrifield replaces Weston Wilson or, who knows, Cave, those guys aren't young either (just younger). Which is really the bigger problem - the Phillies don't actually have any young players in AAA who can help.
My only concern about the money is when you start paying a guy that kind of cash and he's a vet, you also tend to play him more. Harrison rarely played but he wasn't making much (and thus could also be easily released).