The only good reason for Ramon Santiago to be on the Major-League roster is make he doesn't take at-bats away from better players at AAA. In other words, Santiago, if used like the bench-warmer he is, won't get many as at-bats at the major leagues as if he was starting in Tacoma.
But he certainly isn't the 25th best player in the Mariners organization. Santiago has an Equivalent Average of .176, the lowest on the M's 25-man roster. That's lower than Neifi Perez, who was just released from the Giants. If we look at the Major-League Equivalent Average, which approximates what minor-leaguers would do in the major leagues, we see that there are five Rainiers over .215, well above where Santiago is playing.
I suppose the other factor is that Santiago plays short, whereas A.J. Zapp, who's been on a tear recently, plays first; and we already have enough corner infielders on the big league roster. Mostly, I continue to be flabbergasted that we traded the best shortstop in the majors (even if you count A-rod) for Ramon Santiago.