Resistance to status effects is usually of limited utility since it only reduces the time mezzed. Most enemies are obliging enough to break the sleep using their own attacks well before it wears off anyway.
1. Just for completeness sake, resistance to status effects generally does reduce the duration of the mez, with one pseudo-exception: KB resistance reduces the magnitude of the KB. That's because of the peculiar nature of KB, where KB is both a status boolean *and* a separate physics engine effect.
To simplify a bit, KB magnitude determines both *if* you are thrown, and *how far* you are thrown. KB magnitude less than or equal to zero is no KB: if the KB effect is less than your protection nothing happens. KB magnitude greater than zero but less than one is knockdown: you fall down, but aren't thrown. KB mag greater than one throws you a distance related to the total magnitude of the KB effect.
Weirdly, most
melee KB protection had both resistance and mag protection: specifically they had 100% resistance** plus protection. Because they had 100% resistance and KB resistance isn't capped below 100% like damage resistance, just the resistance alone made them immune to KB. The magnitude of the protection was basically irrelevant (it would be relevant to unresistable KB, but that didn't exist commonly).
2. Status resistance was often belittled, but I personally think it did have some value in many cases. For example, most players didn't know that the typical NPC hold was designed specifically to be non-perma. That way a single such critter couldn't just perma-hold a player with no protection and kill them. But the hold could have very high persistance: Illusionist blind for example could keep you held something like 85% of the time (if it hit). But if you faced multiples you could still be perma-held, and if you faced them above even con their holds would increase in duration with the purple patch, and you could be perma-held, and some mezzes *were* capable of perma-holding, like the Malta stun. Having resistances to these effects, particularly but not just when solo, could be the difference between breaking the mez fast enough to survive or not.
Sleeps, as you point out, were generally the exception because sleep was a "fragile hold" - damage broke it, so the odds of being perma-slept or even slept for a long period of time were low.
** Actually, they sort of had 10,000% resistance. This was a vestigial example of the early devs that built the game being either mathematically illiterate or not understanding the game engine or both. The powers set KB resistance to 100, on a scale of zero to one, with one being 100%. So "100" is 100x 100%, or 10,000%. Because there's no real difference between having 100% resistance to KB and having any other higher number, the error was never changed (also the game effectively caps KB resistance to 100% for magnitudes because higher numbers have no meaning***). But if you're ever wondering why City of Data lists status powers as having "+10000% Res(Knockback, Knockup)" and what that could even mean, that's why. Its not an error - at least not an error on City of Data's part.*** On the other hand, resistances greater than 100% *do* have meaning when you're applying resistances to durations. A conversation I had about that subject prompted, I think, the devs to make changes to how status resistances were capped and handed out which significantly increased - albeit not as high as I think was appropriate - the level of status resistance you could get in the invention system. *Eventually* status resistance could have been a lot more valuable than we are discussing here, because we could have been talking about potentially very high values of resistance being achievable by players - well in excess of 100 percentage points****.**** Don't get me started about the difference between % and percentage points.
