Generally a 10 day non-expiring pass is an excellent idea for a trip up to 7 days. For an 8 or 9 day vacation an expiring ticket that spans the entire vacation is probably better, a vacation of exactly 10 days should have an expiring ticket.
The savings you think you achieve may be diminished, possibly greatly, if on your second trip you don't have enough days remaining and you end up buying another pass to go with the leftover pass.
You will need some guesswork and a crystal ball but your number crunching should include this:
1. Figure out your ticket budget for this vacation and also for next vacation using all expiring tickets.
2. Figure out your ticket budget for this vacation and also for next vacation using ten day non-expiring tickets for this vacation.
Disney hints:
http://members.aol.com/ajaynejr/dispass.htm
Warning: Ph.D. in mathemetics required to read what follows.
Another way to crunch the numbers.
Figure out your ticket needs with expiring tickets for this vacation only.
Then figure out the added day cost for a longer non-expiring ticket (Example: four more days added to a 6 day pass cost about $3. each.) and the non-expiration costs ($107.)
For each leftover day count two points. For each leftover plus (for plus pack tickets) count one point. Spread out the non-expiration cost over the point total, for example you have two leftover plusses and the 4 leftover days for a total of ten points. So each point stands for $11.
Since the plus pack yields a minimum of two plusses (on a 1 day pass) the first two plusses absorb the $48. cost so the leftover plusses are valued at just the point count, or $11. each. Each leftover day had its own cost of $3. and two points of $11. each for a cost basis of $25. each for your next trip.
Let's say your next vacation is for seven days. You are better off buying all new 7 day expiring passes and hanging on to your old passes. If you bought 3 day passes instead, intending to use up the four old leftover days, you forego the 4'th new day at $15, the fifth new day at $9., the sixth and seventh new days at $3. each while consuming old days that cost you $25. each.
Do the point count before every 10 day non-expiring pass purchase. In this 7 day vacation you won't go wrong buying 10 day non-expiring passes again leaving three more unused days, and eventually go on a vacation with enough old passes that you don't need to buy new ones.