About half of the fights.
As a general rule, minions should have one or more of the following:
* Ranged attack
* On death power
* Respawning or Death prevention mechanism
* Notable melee attack if melee only
If they are melee only, it's also useful if they do have some form of mobility.
Basically, the problem to avoid is they have to clump up to attack anyone, don't deal much damage, and are hit on 5s. Some published minions have this problem. Some minions work quite well though.
Minions are also a useful mechanic for "collateral damage" enemies, like dominated peasants, forced militia, that kinda thing. In that case, they can be ineffective gladly, as the PCs have a reason not to kill them. I once had some minions do nothing but bull rushes, because they didn't want to fight, but had to - it was actually effective against the players and they didn't want to kill them, cause they were sorta noncombatants.