Personally, I feel the extra money for a park to park ticket is worth it at Universal. I almost never change parks at Disney, unless I'm switching from a park that is closing early to one that is staying open late. We definitely don't do MK, Epcot, MK, Epcot, MK in the same day. However, at Universal, that type of switching is exactly what we do. We always end up switching back and forth between the parks 2 or 3 times a day.
I think for us, we view Universal more as one big park, than two separate parks. The best way I can explain it is that it really feels like you are changing lands not parks. I guess it would be comparable to going to MK and only doing Tomorrowland and Fantasyland on Monday, and saving Adventure Land and Frontierland for Tuesday even though they are right next door. We like having both parks available to us. I would feel like half the park was missing without the park to park.
Here are the advantages:
First, the two Universal parks are very close together making it easy and convenient to switch back and forth. At the back of the park, you can take the train. At the front of the park, you can walk between the front gates. It's about like walking from Adventure Land to Tomorrow Land. In less than 15 minutes you can change parks. No buses, boats, or long rides required.
Second, the weather. If the weather turns bad, hopping allows you to take advantage of all the indoor rides in both parks, so you don't loose park time.
Third, crowds. Hopping allows you to take advantage of popular rides in both parks early in the day and then take advantage of less crowded attractions in both parks later on.
Fourth, restaurants. Hopping allows you access to all the restaurants in both parks. A lot of the time we also go to Citiwalk to eat, and then just enter back into whichever park happens to be closest to our restaurant.
Fifth, to ride the Hogwarts Express between parks you need a park to park ticket. The ride has a different storyline each way.
Having said all that, you can go and have a great time not switching. Especially, if you don't know what you like yet and want to try most everything anyway or have EP tickets so you aren't worried about crowds. We've just found for our family that park to park is worth it.