I noticed this issue when I was fooling around with FASA. Some of the old capsules don't have reaction wheels. I noticed this interesting fact only when I realized that Mechjeb wouldn't point the ship towards the circularisation burn node.

However, rare this instance may be, but hardly unheard of. I think it would be desirable if Mechjeb were prepared for this contingency. I therefore request the following feature:

When ordered to execute a maneuver node and no currently active reaction wheels exist on the vessel, Mechjeb should activate RCS.

The Attitude Adjustment has a "RCS auto mode" that activate RCS when needed.