To me, it depends on the goals surfacing in one's 2yr - 5yr travel forecast. If the coming years are "soft" (w/out any specific plan) then borrow. However, if one has a big family gathering forming on the horizon ... then I'd rather conserve my natural points and follow a short-term plan buying OTUP.
From what I understand, the OTUP would go back to your current UY. They cannot be banked or transferred out. Not sure if they "show up" online or if you must phone MS to use them.
We have owned since 2007 and had points transferred a couple of times from other owners. In the last couple of years we have used more points and found the need to buy OTUP twice. It is a nice option when you use more points than your annual allotment and don't want to add on.
Members since 2001. Using OTUP for the first time this January. First WDW trip w/o the kids. Going for our 24th wedding anniversary. We have APs and had airline credit so we wanted to squeeze another trip in but we didn't want to use "regular" points as our kids would not be happy with a shorter trip this summer.
Given our new "Companion Pass" from Southwest Airlines ... I might see myself dropping in at WDW more than expected through 2018. As our Oct 2017 points are already fully consumed, I might have to seriously consider borrowing, OTUP, rental/transfer points, etc.