Plus Day v. Park Day
Yes the plus options at one time expired but is now forever until used. Say you have a 5-day plus 2 ticket. You could work it for seven days.
Day 1 - Blizzard Beach
Day 2 - Epcot
Day 3 - Disney Studios
Day 4 - Typhoon Lagoon
Day 5 - Animal Kingdom
Day 6 - Magic Kingdom
Day 7 - (Repeat any park day combination)
Of course it is a "hopper" so the park day combinations could be even more complicated. Where Disney makes the $$ is when you do a park in the morning, break in the afternoon to the plus feature, and back to a park that night. (I didn't mention PI since it is a night attraction and it would normally be used on the same day as a park. However, you could do PI on the first day if you have a late arrival and don't want to spend a park ticket on a partial day.
One hint I have for keeping up with those plus features you don't use. At one time I had a 5-day 4 plus hopper from Wal-Mart. I marked the back of the ticket with (4) (3) (2) (1). As I used each plus I "X"ed out the highest number. So when I knew I'd used all the park days all I had to do was look at the back and see if any plus features were left. You probably could do the same for park days by marking it somewhat differently, [5] [4] [3] [2] [1]. No more guessing what is left on a ticket until you can get to a strip reader at WDW.