Martin represents someone with upside, someone who could be a regular at a tough position.
Much rather roll with him and see if he develops vs signing Iglesias, a guy you know won't hit. Not to mention, you have Iglesias for a year compared to a potential of 6 years (or more) with Martin. It's pretty ridiculous to suggest otherwise.
As for Granier vs Martin. So what if they are the same player? If they both develop. you have a trade chip or one can move to second. If one develops, you have insurance. If neither develops, no big deal.
If Martin doesn't show a big difference with the bat this year, he may not get picked. Granier needs to show something with that bat. You don't just bring him up because you think he can field. He still needs to develop and while I think the term "rushing" is stupid and most don't understand how to properly use it, giving a very flawed player almost zero MiL time to develop is just dumb.
Now, if Granier is doing very well with the bat this year and Martin isn't the answer, I do consider Granier as an option later on this year or, more than likely (for service time reasons), next season.
But right now? You take the guy that is obviously the best player available, plug him in and see if he is a late bloomer. Signing Iglesias and playing him over this kid is a dumb move with zero merit.