The reason the Church is happy to have it read is because it is another story to verify some of the stories already in the bible...
However - as you noted - the church will not add it to their own studies nor will they have it used during services...
Let me put it this way - When Vincent(a close friend to my brother) died 3 years ago, the priest-father, refused to read the gospel as requested by the family. He told them that since it was not in the bible, he was not allowed to use it during the services. They were just a little upset(sarcasm intended), as he died during gang-related activities and the family wanted to express the possibilities of this gospel to other youngsters...
The only acknowledgement I get from the church on this gospel is that it agrees with Acts 7:48, it also exemplifies some of the in-depth teachings that the disciples left out of their books, some of the teachings they paraphrase ...
The reason it would destroy the church is because people would not need a priest to break bread for them, take their confessions, christen their children, baptize the re-born in faith, bless their home or being, and teach them what they need to know from a service...
Have you ever attended a Catholic service, if not let me fill you in...
It is a programmed service, with pre-determined readings(set for centuries to come). If you look in the back of some bibles it will tell you what gospel will be read and on which date, along with the stories to go with the theme for that week... You read the gospel the same time every week, break bread the same time every week, stand, kneel, and sit the same time every week, pray the same prayer every week...
Fact is, the reason the church is so strong is because masses like set guidelines... They get programmed at an early age and feel comfortable enough with it that they remain droids for life...
Something as simple as telling a person they don't need a priest to confess to cracks the foundation of the Catholic hierarchy... Thus they take the good from the gospel of Saint Thomas and tie it in to the bible, but they don't hand it to you to read, they don't discuss it with you, they simply tell you you can find a copy of it somewhere and read it... and when you ask questions you get asked to leave...