Librarians, Can you really put your faith in them, they can turn on you in a heartbeat, blow up when you need them the most. Well Psykers in general are fickle beings. Most of a nice LD of 10, but if you roll a double 6, or a double 1, you get hurt, and with most psykers, you have no save what so ever to protect you.
The standard state line for a Librarian is WS5 BS4 S4 T4 W2 I4 A2 Ld10 Sv3+ which is pretty so-so as Space Marines go, one point 1 extra of weapon skill, wound and Leadership than your Joe Sergeant. Not someone I really want leading the force at the front, more of a back field character, which is good expect the fact in most armies you pay a pretty penny for maybe a couple re-rolls and maybe a nice blast.
Don’t get me wrong, I like the using them, especially the Sorcerers from the Chaos Space Marine Codex, and the Librarians from the new Dark Angels Codex being able to grant a 4+ invulnerable save to them and having them starting at a low point cost with decently priced mastery level increases. They will in the end still cost you a lot, but you will get more use out of them and have more fun.
Having to roll though to get powers and to use them, and having the risk of blasting your prized Psyker into little glowing pieces is not something I look forward too. You run great risks when playing with Psychic Fire and if you build your army around it then you can run even greater risks of your army folding under itself because of the random dice rolls. Use them or not, you run the risks of them blowing up in your face. Then again if your playing Orks, then your guy is as likely to blow up as the guy standing beside him. Head explosions gotta love them.