How is Temper being single target only not a total failure, especially after SE going on about how RDM was going to be master enhancers?

Of themselves? Yeah, because that's oh so useful in a party situation. It's not the Accession clause (that I don't mind, it's RDM only after all so it shouldn't AoE) it's the self-cast only that's complete nonsense.

So hurray for yet another trash spell to "help" frontline RDM in lieu of addressing RDM's actual problems in that area.