Better star images means that the autoguider will lock on and stay on. Fourth, with a guide scope it is much easier to guide on comets as they are moving relative to the background stars. Finally, a guide scope can be used as a secondary photographic instrument, often giving a wider field of view than the main scope. The main scope can the act as the guide scope giving you near perfect tracking. One nice thing about off axis guiding is that you are guiding using the same optics that you are using to photograph with.
Flexure is when one optical system moves relative to the other.
If you have too much flexure, your autoguider may seem to work perfectly and keep it's guide star perfectly centered with the field of the guide scope, but because the guide scope has flexed slightly relative to the photographing scope the stars will trail and ruin the picture. The only fix for this is a very ridged mount for the guide scope. The second is a problem with Schmidt-Cassegrains in that that the optics can shift slightly depending on the orientation of the scope. This translates to the stars appearing to move as the orientation of the scope moves, and thus your photograph being ruined. With an off axis guider the autoguider will compensate for this movement, but with a guide scope the mirror movement can not be compensated for.