What's the point of you being able to emulate having a slot if the emulated slot isn't somehow usable?
Depends on the item, I'd imagine. I'd also argue that emulating having spell slots is less likely to be important; I can't think of any items off the top of my head *other* than the Runestaff that requires them, and the Runestaff only requires them so that you can expend them. If there were an item that gained increasing power depending on the highest level spell you could cast, then emulating such slots would be useful. For a runestaff, though, it is not useful.
Keep in mind what the purpose of a runestave is - it's an item that is supposed to allow a sorcerer, through an item, to increase their spells known, and it's supposed to allow a wizard, through an item, to gain a bit of spontaneous casting ability.
Would you allow someone, with a DC21 UMD check (and a fairly inexpensive item), to cast Cure Light Wounds at will? That's what the OP is asking for - unlimited spells.
Normal staves and wands have charges; runestaves use your spells-per-day as their charges.
Gaining a familiar is a ritual that takes 24 hours and costs money.
But emulating the "Familiar" class feature is a DC 21 UMD check. The fact that the ritual takes time and costs money is immaterial.
I do not know of a spell that does this. I do not know of a magical device that does this. Ergo, I don't see how UMD pertains to familiars.
Consider an item ("Keoghtom's Bag of Puppy Treats") that allowed anyone with the Familiar class feature to affect, with a command-word activation,
Enlarge Animal 2x per day at CL whatever, but it could affect magical beasts.
The item is obviously designed for wizards to "buff" their familiars - but a rogue could use it on a trained Unicorn mount with a UMD check, DC 21 for Wizard 1 or Sorcerer 1, to emulate having the Familiar class feature.
But, additionally, Sorcerer is not a class for which the particular ability ("emulating a class feature") is going to come up very often.
That's just not fair. You are not emulating a "fake" slot. You are emulating a real one.
On the contrary - "This skill does not let you actually use the class feature of another class." You can emulate having slots, but
you can't use them.
If your relevant stat isn't high enough, you have to hit another UMD check for that every time as well.
No, you don't. You only have to emulate a stat score for spell completion magical items, like scrolls.