I recently attended a few evening classes at a local college to get a bit of practical welding experience. I was brazing for a couple of classes and can clearly recall the instructor saying that the college's Formula Ford chassis (which he had to repair regularly!) were a brazed space frame construction. A brazed joint is allegedly 'stronger' (than a weld)(in stiffly sprung applications) due to ductility of the joint. Note that this was an evening class for the general public, not engineers, so i was never going to get a scientific answer, but having thought about this it makes sense:
A perfect welded butt joint in pure tension would be stronger due to the higher yield strength of steel. However, in reality, and when joining tubes of all different angles and sizes, the stress concentrations and thermal stresses introduced by welding would reduce the ultimate strength of the joint. Also, with a correctly triangulated frame, i would expect lower stresses with brazed joints - the relative stiffness (elastic modulus) of the brazed joint and steel tubes will alter the loading such that, for a given amount of chassis bending or torsion, the cross-bracing members will see higher tension/compression forces and the joints will see lower moments. Basically, a brazed frame will be closer to a pin-jointed structure than its welded equivelent.
As mentioned by earlier posts, there are several benefits to brazing - reduced cost and heat-treatment. But for an inexpereinced FSAE team or individuals, brazing is particularly well suited because it's so quick and easy - there's no UV arc or potential spatter so you can work in comfort and see what your doing. Also, probably most important, particularly for a prototype, is that if you decide a tubes in the wrong place then you can just heat the joint up and remove the tube - easy and far less aggressive than cutting out tubes and grinding joints down.