Underdrive pulleys: not using Alt pulley?

In general, any modification to your car is going to create more wear and cause your car to sacrafice some sort of functionality in the name of extra power. Yes, you will wear your alternator at a higher rate than with the stock pulley. How much? Not enough to be all that concerned about it.

Will you lose some charge when you use the underdrive alt pulley? Yes. Will it actually cause problems? No. Not if your igntion/electrical system is in good shape and running fine prior to installing the pulleys. I've run all three underdrives on both of my SN95s and had NO problems with cooling or charging. Foxbodies may incur some somewhat significant problems because their alternator is weaker but you should have no problems. If you do, look into what part of your electrical system is causing it (battery, alternator (not the pulley), etc)

Will you lose some potential power by opting not to use the underdrive pulley? Yes. That pulley is part of the package for a reason. If you dont use it, you will not get the extra power that that pulley would have been able to provide or free up.

I would go ahead and put the underdrive pulley on and if you run into problems THEN put the stock pulley back on. Likewise, your car will not flow as much coolant with the underdrive waterpump pulley. The one thing you need to remember when modifying your car is EVERYTHING IS GIVE AND TAKE. You arent going to get power without sacraficing something else whether it be emissions, fuel economy, top speed, torque, temperature, charge, etc.

Good luck
Chris