One advantage of HOPPER is that on your Animal Kingdom day (AK closes pretty early), you could "close" AK and then HOP over to, say, Epcot for dinner and IllumiNations that evening.
If, say, Fantasmic! got "rained out" on your MGM day, you could HOP to there on the day that you were at another park, and catch the amazing show.
If the "entire population" of Hukalockistan poured into MK on one of the days you were there, and filled all the queues and took all the FP's, you could hop to Epcot and escape the crowd.
If you were at DHS and wanted to eat at, say, Canada in Epcot (like WE love to do) you could ride over on the boat, use your Hopper to get into the International Gateway, dine on CHEESE SOUP, then retrace your path back to MGM for more fun!
We nearly always park-hop... virtually every single day.
Hopping can be a valuable asset, and you don't need to buy it unless or until you NEED it!
How do you know?
What should you do?
Simple:
You don't need to buy the "Hopper Option" UNLESS and UNTIL you need it.
If you are already at one park and decide that you'd like to "hop" to another... right then,
just walk up to a ticket window, add on the Hopper option (costs the same price if you buy it before or DURING your trip) and you're ready to hop (that day, and any others, for the rest of your trip.) You must make this and all other upgrade choices within the first 14 day of when the ticket was first used and no later than on the day that you use the ticket's last asset.
If you go and NEVER need to hop, don't buy the upgrade, and you can then spend all those "extra" bucks on more WDW souvenirs!
The cost of the Hopper option now varies... as of the last price increase (Feb, 2014.)
If you upgrade some tickets by adding "days" you might also have to increase what
you pay for the Hopper option on those tickets.
Adding the Hopper to a one-day ticket costs $40.
Adding the Hopper to a two-day or three-day ticket costs $50.
Adding the Hopper to a four-day thru ten-day ticket costs $60.
SPECIAL NOTE:
If you already have the Water Parks Fun and More option on your tickets,
adding the Hopper is a one-time, one-price fee of just $26 per ticket.
All prices, plus tax.