I think Soul just feels inadequate since he constantly talks about being a cool man, there is really nothing cool about helplessly watching your partner get hurt when she is incapable of wielding you.

The Thompson sisters proved that they can handle themselves without kid and Tsubaki can transform into various weapon so she is handy for various situation. Soul is the only one so far who is probably completely helpless if his partner isn't around or is incapacitated so right now he is really a tool rather then a partner.

It would be really interesting to see Soul learning some new moves that will help him fight along side Maka in a synchronize manner