I disagree on both counts.
Adding Coll-o to the squad could only damage Gilo's confidence; adding him would seem to Gilo, and to all the commentators and media, that there was a question mark against Gilo's name. Gilo's problems are mainly mental -- remember how he was saying a year or so ago that he was thinking of retiring from international cricket, didn't think he had the goods... With soemone like that, you have to build him up, not knock him down. Putting Coll-o in the squad, even if he wasn't used, would have a huge negative effect on Gilo, even if he was used.
As for the quickies, remember that Flint-o was getting surgery just a few months ago. Jon-oS was out injured nearly two years. Harm-o, to me, still doesn't look fit for a full five-match series.
Having a back-up in the squad makes immense sense, in case someone breaks down. The selectors are saying that Trem-o is next in line, and deserves to be there as such -- it's a huge validation from the selectors that he's important to them, and keeping him in the squad will give him the confidence he would need if someone does break down and a replacement is needed.
It's been years and years since one could say it, but the selectors have got it totally right this time. Well done!!!