To address your points:
At $4 a year for two years you have a reliable defender who can hit for contact. Although he isn't a big bat at the hot corner he does provide enough value on defense to be worth the contract he was given in my opinion.
One thing that Dipoto has tried to do is build depth at every position so that if a player is injured we have a better than average replacement player to take over during the interim time that our main player is out of action. Lucho still has minor league options so it makes more sense to give Callaspo a short term contract at a reasonable price until either one of Lucho Jimenez or Kaleb Cowart becomes Major League ready. If one of the latter becomes ready sooner then we could possibly trade Callaspo, with his reasonably valued contract, to another team for a piece or pieces that could fill another area of need on our team.
The depth chart looks like this: Callaspo, Jimenez, Cowart.
Izturis wanted to play full time and he probably wouldn't have done that for the Angels.
Haren and Santana both had velocity drops in 2012. Velocity drops have been proven indicators that an injury exists or will happen in short order. Additionally Haren and Santan threw a lot of splitters and sliders, respectively, which also have been proven to damage a pitchers arm over a given period of time and with a given percentage of use during a season. Although the Blanton signing hasn't started off well I think that Dipoto wanted to take that chance over the clear warning signs regarding Dans and Ervins arms.
Anibal Sanchez would have been a potentially good signing, I agree. Lohse came off a great season but his peripheral numbers showed a completely different pitcher and I think Dipoto was wise to stay away from him. And he would have cost us a first round pick. I'd rather have a more proven, upside talent in Hamilton and lose that pick over a fringe pitcher coming off a career season.
$4 million per year is nothing for this club and it gives us needed depth. Callaspo is of course not the ideal player you want at 3rd but he isn't the worst one either.