Thrust Reversers are not considered in RTO performance for the very good reason that they may not be useable. If you are rejecting the take off because you've lost an engine, and if the runway is wet/slippery with crosswind etc then controllability may be a serious issue. You might only be able to apply idle reverse on one engine at best (in the case of a twin).
It therefore makes perfect sense to consider this worst possible scenario during training and emphasise the order of actions to take maximum advantage of the reliable retardation systems you have. That means:
1) Thrust Idle
2) Brakes on (simultaneously)
3) Speedbrake deployed manually (have a look at the penalties for spoilers u/s for take off!)
4) Reverse Thrust (if available)
IMHO.