The last Pluvian has two things going for him - a fairly high health stat and reduced cost for players with "Guardian" identities.
You're probably not going to include him in any non-Guardian's deck. In a Guardian's deck you can do a lot worse than a 1/1/4 ally for 2 resources. He's a valid inclusion in just about any Guardian deck, but also an easy choice to cut if you need to trim cards.
With 4 health, you can probably get two blocks out of him, or get a few low power attack/thwarts out of him.
He's a very solid inclusion in Leadership decks that leverage the Guardian trait on your allies, since his cheap cost and high health means he's easy to keep around, and a high priority for the Energy Spear in Aggression decks for the same reason.
On a side note, I was a big fan of the original Guardians back in the 90's, and I'm a little disappointed that Marty's depiction in this game isn't particularly interesting.