Does the parking brake provide additional deceleration in an abnormal situation?
This is probably aircraft dependant, but I can give some examples for the airplanes that I have flown and will be generally applicable to transport category aircraft.  Understand that it won't cover every airplane as an absolute rule though.

In general, these types of airplanes have an antiskid system which keeps the wheels from locking up during heavy braking.  If they were to lock completely up, the tires would likely blow, greatly decreasing deceleration and making it more likely to run off the runway (either at the end or off a side).  

Parking brakes often have multiple settings because of this.  One setting will be one which can be used when the normal brakes have failed, and have a relatively low brake pressure / stopping capability compared to the normal brakes in order to protect the tires.  Using this setting will stop the airplane eventually, but you are better off with the normal brakes which have a higher brake pressure with antiskid, and better deceleration.

Another setting would have a much higher pressure, intended to keep the wheels from turning (while it is parked), and this would be undesirable while attempting an abort.

In the end, I will say what I usually say:  Follow the guidance in the AFM.  If it says to use the emergency brake when you abort, then do it.  Most likely it will not (because of the above reasons), and you should not do it.  If you take it upon yourself to go against their guidance, you become a test pilot and are either trying something that hasn't been done before, or worse yet, something that has been tried and has had an undesirable result.