Will somebody tell my mages that it's not a good idea to...

[Wrana]

That they shouldn't be cast if he's the only commander is true. As for the first part - it actually isn't all bad as this allows the mage in question to have the benefits of spells cast by other Communion Masters (and if it's Summon Earthpower or Power of the Spheres... quite nice, I'd say) and still channel some fatigue to Slaves. This isn't for all to do, of course, but for a part of communion it could be useful (and extra Slave can help in increasing power of Masters if you have no surplus slaves).