Our team currently has 5 operational Jaguars, but we are in need of a 6th motor controller for a reloading mechanism on our robot. We do not have another motor controller but we do have a Vex Spike relay module. Do you think it is possible for us to use the Spike to control the motor that is operating the reloading device?
If you scroll down to Rule R52 in The Robot section of the game manual, it clearly spells out the answer to your question. 
According to R52, as stated in the last post, it is allowed. I think it might end up making some code difficult to understand because you will be using different driver code, causing some inconsistencies.
As stated, yes you could use a Spike to control your reloading mechanism, provided the motor you intend on using is covered by the rules. As well, I caution you against using a Spike as a motor “controller”, since it’s a relay and simply a high tech on-off switch.
What he means is not to try and “pulse” the Spike like a Jaguar and Victor by switching it on and off rapidly for speed control. Set it on when you need it and leave it until you’re ready to turn it off.
Just to make sure this is clear: Spike Relays will not be able to switch fast enough to act like a normal speed controller. You should not try to do that. The switch in spikes are actually physical switches which take time to change. Constantly switching causes a strain that it can’t keep up with. And even if that wern’t true, is most cases in your code, loops only run every 20ms, which is much too slow to act as a PWM speed controller.
Thanks for all the responses. We have figured it out. Much appreciated.