An interesting concept but a few things you might want to think about:
1. How powerful are these spirits? If very powerful, then it reduces the possibility that someone would summon them, knowing that they are uncontrollable. If their power is average, then this is less of an issue and things might work out.
2. The ease with which spirits can be summoned might be a major motivation for non-mages to summon them so maybe a story on the first "diastrous" attempt by ordinary ppl which might be fun. However, still need to map out what ppl summon these spirits for, given that they are hard to control. Some of my thoughts are: casting magic on them and making them solid and then running/teleporting out of the room so that the spirit will turn malicious and attack whoever still stays in the room (for a minor spellcaster); some might summon their loved ones back just for comfort, even knowing the consequences; reckless ppl summoning them to do their dirty work etc.
Hope my comments have been of help