I would have two problems with the whole developing Tata in middle relief idea, which makes this a TOTALLY different situation from Johan Santan or even Zumaya.
1) His walks are a little higher than you'd like to see right now. He's walked more than he's struck out. Not a good sign for a pitcher, but he's not too far off, ie. Franklyn German levels.
2) He was basically the last guy out of the bullpen getting very infrequent work. He didn't have a significant role like Zumaya does. He needs to pitch more MEANINGFUL innings on a regular basis and he will do that in Toledo.
It's not at all about him hurting the team, which he has not done. It's all about making sure he gets the work he needs and he was not doing that as a garbage-time guy out of the pen. Don't forget, Tata was originally expected to start this year in AA, but was a surprise member of the big league team when Dingman went down with injury. He was definitely impressive and I think he'll definitely be back sooner or later.
As far as justifying the Colon trade by bringing him up, I can only see that as speculation right now. He was effective, albeit in limited time in AAA, with no help from his defense leading to quite a few unearned runs. He certainly has a lot to prove, though.
And, yes, Zach Miner is proving to be quite solid and then some. He has a 2.03 ERA in 41 IP with 31 K's and 17 BB's and only 31 hits allowed. 5 wins and 0 losses to go with it. He's been one of Toledo's bright spots this year.
Honestly, I am not a Grilli fan and would like to see him sent down soon, but someone seems to like him, so I don't expect him to get sent down anytime soon. Also, Grilli would have to clear waivers, not that I'm sure he'd be such a hot commodity.