Like how many apps can take advantage of it? Which ones take most advantage of it?
Thanks.
No Pencil:
Receive form in email.
Print
Walk to printer
Pick up
sign
scan
return to desk
email
Or (with pencil)
Open PDF
Sign
Email
Or
take photo
sign
email
It's not a must, but that's a killer use case for me. Less paper wasted. Less stolen print jobs. Less time spent wondering: "Is, M880-East the closest printer to me, or is it M880-West?". Then finding that the printer jammed, or someone else is printing a 500 page document, etc.
Plus, drawing and screwing around with it is fun. Novelty: sure. But i haven't actually done any real art work for YEARS i until i got the pencil.
Also, in iOS 11, it will index my handwritten notes. I am constantly losing pieces of paper with notes, scribbles, etc. on them. I find it more difficult to lose my iPad. And even if i do, the notes are cloud synced. And indexed! I can search with a search box, rather than 2-3 different note-pads and loose bits of paper on my desk! Plus modification date and time automatically recorded! And they are with me wherever i go!
Sure, some of that could be alleviated or made redundant if i was less lazy about recording what I do. But I am what i am. I'm lazy when it comes to organising my life, and that's unlikely to change. I've learned to accept this. The iPad and the above pencil related features can really help me with this.
How important all that is, is up to you. But all of the above is pretty much not possible (or rather, annoying and more awkward than it needs to be) without the pencil.
I'm also a network admin/architect. So i use the pencil for sketching out network diagrams, rough maps of physical locations, etc. If you don't do that, fair enough, but for me that is another major win for the pencil for me.
Also meetings... sketching or taking notes is easier for me than using the touch keyboard. and i find it less rude if someone is writing than if they have a screen up that they are typing at. Small point, but relevant, imho.
Is it a "must-have"? No, probably not. But i do think it's something that, if you're spending the money for an iPad Pro - you may as well buy the pencil as well.
If you're NOT already in the iPad pro market and are considering say, an iPad 2017 vs. an iPad Pro + pencil, then that's a different scenario (the financial step from 2017 iPad to iPad Pro plus Pencil is HUGE). But if you already decided on a Pro, get the pencil. There's a whole slew of use cases for the Pro that you just can't do without it.
edit:
Also. If you have the Smart Keyboard, using the pencil as a pointer for the screen can sometimes be helpful to reduce the amount of time you spend with "gorilla arm" to paraphrase Steve Jobs. You can use the pencil as a finger extension somewhat. So you don't need to hold your arm "up" to point on the screen so much.
Again thought its more of a nice to have. But i do find that there are so many "nice to have" use cases that I think that (again) if you're in the iPad Pro market, you may as well spend the little bit extra on the pencil to unleash all of those "nice to haves".