I think it's silly in the regards that we have precisely one datapoint about the sort of environments in which life may exist, which is pretty terrible in terms of making any sort of definitive statement. I'd much rather they keep their options open, check out a wide range of environments, and just look for signs of "things that are hard to explain", whatever they may be. "Hmm, this body has both a strong oxidizer and a strong reducing agent in its atmosphere - how is that happening?"
I'm not saying "check planets in random order" or anything of that nature. Just that I don't think it's critical to obsess over being sure to examine them in order of "earthishness" from highest to lowest. We need to be looking at a diversity of worlds.
Heck, we don't even know whether the surface of a body is the best place to look, most life in the universe might be in sub-crustal layers for all we know. Certainly would partially help explain the Fermi paradox, if it were such that we rare "surface dwellers" have a far easier route to the cosmos than something that needs to be under gigapascals of pressure to survive and whose radiating transmissions, if any, would be blocked by their planet's crust.