I don't think anyone has actually tried this with a limited length pass, but here is what I fully expect...if you make FP+ reservations, but don't enter the park that day:
- You don't lose a day on the pass. They would have LOTS of angry guests if you did.
- You don't lose a day of FP+ reservations. However, you won't be able to make another day's reservations until the ones that were missed completely expire at the end of that park day. That is, if you have a 5 day ticket, you can book 5 days of FP+ in advance. If for some reason you don't make it to the park the first day as expected, the FP+ reservations you have will expire at the end of the day. After they expired, you can then make another day's FP+ reservations to bring the total back up to 5 (or whatever number of days remain on the ticket)
I don't see this as any sort of problem even with non-expiring tickets, because you can only make a limited number of reservations anyways. All the speculation about people eating up all the slots on speculation that they _might_ go to a particular park for the next X days I think is highly, highly unlikely.
I am excited to try it during my next trip

And am still considering purchasing the 10day passes while they still are available...does anyone have 10day non exp and find them worth it?
Depends on what your trips generally look like...do you do lots of one or two day visits? Or do you stay for longer periods of time? Do you take more than one trip a year? Do you do the water parks/DQ/etc.?
The biggest problem with them is that you need to fit two or more trips completely within the 10 days. If you have to purchase an additional ticket to complete a vacation, you generally end up spending more than if you just purchased the right length tickets. Although if you plan to make lots of trips in the future, a valid method of using them is what I call "chaining" the tickets - when you need additional days, just get another 10-day NE ticket, and start using that, and then another, and then another...
If getting a No Expiration ticket, I highly recommend getting the WPFM option (at least on the first one if chaining), as those options don't expire either, even after all the days are used. On a 10-day NE WPFM ticket, you get 10 water park/DQ/etc. visits as well, which can last some people many trips...even when they now have switched to other ticket for regular park admission.
Also if your trips can fall within a calendar year, an Annual Pass can generally be a better deal than a NE ticket. APs come with free parking at the parks, free park hopping, merchandise and dining discounts, etc.
In general, if you park hop and would visit 12 or more days a year (been a while since I've checked the exact number), the AP is the better deal not even including the other perks.