Brakes are primarily for braking and engines are primarily for accelerating.
Approaching a corner I use the brakes to slow down whilst shifting down to arrange my RPM correctly to maintain and eventually power me out of the corner at the appropriate time with as much grunt as required.
Too higher gear and you open the tap and it wont accelerate out (everyone else just sails past or runs up yer ass) and too lower gear and it just revvs the #### out of it, makes a lot of noise (and you are now alone on the road) so the skill on down shifting into a corner is based around an eventual power application. The advantage (or byproduct) of shifting down is that you can also use some engine braking advantage and this can be a nice bonus. On a lot of bikes this can be an excessive amount that will give you a rear wheel lock up, hence the slipper clutches that are fitted to such types. On a Sprot bike its your front wheel that does all the retarding in a straight line and the back brake and engine braking augments it on a lightening (as in getting light on the road) rear wheel. However it is a common technique to use the braking effect (brake and/or engine) on the rear wheel to assist with stability and directional control during high speed turn entries or, in some cases, patching up an over-cooked turn whilst retaining a suitable lean angle as the front brake will tend to stand you up. Tricky stuff using any sort of a brake whilst leaned over in a turn and often ends in tears..So I gues the moral of the story, or the safest option is to get it all done (or at least the majority of it) before you roll into the corner. Somewhere around this site I see a Signature (can't remember where) that says something like "Enter the corners slow and exit fast" which makes good sound sense to a person that rides on the public roads. As always the application of any sort of rear wheel braking needs to be treated with respect as it has similar effect to that of pulling only the handbrake on your car and if mishandled the rear wheel will loose grip (skidding wheels have less friction) and eventually attempt to overtake the front.