All math aside, I think how "fun" this is is going to depend on the build of the party.
For example, a friend of mine just built up a 3rd-level Warlord who dishes out about 4 saving throws per encounter:
She's got the feat which lets her grant a Save along with her Inspiring Word Minor
She took the L2 utility which grants a Save as a Minor
She took the L3 encounter which grants a Save on Hit with a Standard
As a DM, I'd have no problem tossing a "Stun (save ends)" or "Dominate (save ends)" at somebody else in the group, because it would give her something awesome to do with her turn.
On the other hand, if a party doesn't dish much extra saving, and is having to rely on the Standard action "grant an extra save", yeah, I do think that's much less fun, with the party likely to get quite frustrated by it.
I'm certainly not arguing for a "Stun at-will, stun interrupt" monster, that's just awful, and I can't imagine it being fun in any meaningful way.
But a "Stun (save ends) on crit" monster, or "Stun (save ends) as encounter power" monster, maybe even up to a 5-6 recharge power - that can certainly be fun.
For example, a friend of mine just built up a 3rd-level Warlord who dishes out about 4 saving throws per encounter:
She's got the feat which lets her grant a Save along with her Inspiring Word Minor
She took the L2 utility which grants a Save as a Minor
She took the L3 encounter which grants a Save on Hit with a Standard
As a DM, I'd have no problem tossing a "Stun (save ends)" or "Dominate (save ends)" at somebody else in the group, because it would give her something awesome to do with her turn.
On the other hand, if a party doesn't dish much extra saving, and is having to rely on the Standard action "grant an extra save", yeah, I do think that's much less fun, with the party likely to get quite frustrated by it.
I'm certainly not arguing for a "Stun at-will, stun interrupt" monster, that's just awful, and I can't imagine it being fun in any meaningful way.
But a "Stun (save ends) on crit" monster, or "Stun (save ends) as encounter power" monster, maybe even up to a 5-6 recharge power - that can certainly be fun.