No question Janish is the best, by far, defensive option of the three guys you mention. Plus, he runs well. I agree that we're talking about the 24th/25th guy here. I'm just saying Pena had a nice little run (and he was regressing when injured), but he filled the dance card nicely for the first few months of the season. Is he the reason we're in first? Never said that. Said he was a nice spare part.
Janish will stick around baseball for quite awhile. Great glove and when you see guys like Kozma clipping along at a sub-.600 OPS rate, I can see a situation where a team could conceivably give Janish a shot at starting. Janish's problem--like it is for a ton of guys--is that they have to play regularly to hit, but they can't hit enough to play regularly.
I guess my bottom line as it relates to this topic is that I'd prefer Janish to DeRosa.