Are stick disarms in kali worth spending a lot of time training?

Disarms are certainly an integral part of any weapon-based martial art.  Bottom line: if you can take your opponent's weapon away from him, he can't hurt you with it.  But it is only a small part of the system, not the primary focus.

That being said, disarms are incredibly difficult to pull off successfully at full speed - the stick simply won't be there anymore by the time you try to disarm it.

They're definitely worth the effort to learn how to do them, but accept a certain dose of reality that you won't be able to "look" for them at speed - they'll occasionally present themselves and it's up to your skill and timing to take advantage of those small windows of opportunity.

Typically when you think of disarming, it refers to maneuvers that involve directly manipulating the weapon hand to physically remove the weapon from it.

However, consider another type of disarm - simply striking the weapon hand.  A broken hand won't be able to hang onto that weapon.  (To be sure, this rarely works in a sparring situation because the hands are protected.)  You could also consider striking other targets to be ways to disarm - hit a guy in the head enough times, and he's probably going to drop his stick.

The general rule is not to "try" for disarms.  The saying, "Don't ride a dead horse" applies well here.  The key is to have an SOP (Standard Operating Procedure) for disarms.  If you're in the middle of a disarm and you feel it failing, then start hitting again!  Or maybe move to a throw.  If you get the throw successfully, you can work the disarm while the opponent is on the ground.  Just don't keep trying to finish a disarm when it's become clear that it isn't there anymore.

Something else to think about is this: who do you anticipate disarming, a trained kali fighter or a (probably) untrained random assaulter on the street?  An untrained person will not swing the weapon in a disciplined manner and won't have an understanding of the mechanics behind disarms and therefore won't know how to respond in order to counter.  So taking a weapon away from an untrained person should be in theory easier to accomplish than disarming a trained person.

So to answer the question, at the very least spend as much time as you feel is necessary to be comfortable doing them at speed and under pressure.  If you can confidently do one or two disarms from any angle, that should be enough to deal with most situations.