Well, it's not 100% harmless. There are other programs which spawn threads which are of low priority (hence, done in "idle" time). If you install RC5, or any other program that uses "idle" time for that matter, those threads will likely not be able to perform there tasks. This happens sometimes in rendering farms.
Plus, a lot of computers have @$#%^( cooling, and would die if they were real world "stressed" (Most people open word, a web browser, and maybe email, and they're set.....which isn't stressing a system).
Power consumption. Some people will (or already do) have massive power bills. Some companies, if they were smart, would make sure to either use an OS with the hlt command built in, or to use a program which employs it, which can greatly reduce power consuption. This also has the added bonus of not spitting out lots of excess heat (and we all know that electrical heating is inefficient use of electricity). Plus, some places have to use AC already. It saves power not only because the CPU's aren't wasting as much electricity, but also because their AC doesn't have to work as hard.
On laptobs, if battery powered, users may want to have their power managment turned on, and use the hlt command to conserver battery life. RC5 and others can be made so that they don't run while on battery power, but not everyone remembers that. So save the laps of laptop users, and don't have it run while on battery power, and help them be able to do their work longer on one charge
There are MANY
realistic reasons why one should get permission, aside from ethical ones (though they do coincide).
Oops...I gotta go take my chem exam! Ack! Later!