Reason:
1,Imagine Breaker may be the only hope for Morrigan Aensland's victims
2,Kamijou Touma has better precognition than Morrigan Aensland,although seems slower,but he has IBed attacks from opponents at similar or even higher level.
3,both seems good at "charming",and Touma seems even better
4,considering IB is bonded to Touma's soul,and anyone who attempted to absorb IB suffered a lot from their attempts,it's not a good idea to absorb Touma's soul.Of course,cutting off his right arm also means great risk.
Touma seems a lot weaker in surface attributes,but stronger in battle records,as he has defeated a lot of opponents with higher ranking,and being favored before all these battles by Aleister Crowley.