On a week long trip, with six days three park days (a park every other day) is all I want to do. And the way Disney prices the tickets there is no material savings till the fourth day which is basically free if you choose all Disney parks. Some years too, I only go down to Orlando for two days (one day Disney and one day SeaWorld). Also, the non-expiring ticket premium you pay is really high. This used to be no cost, and I have NOT bought a multi day Disney ticket since they started charging for this, as the saving incentive just isn't there for me. This has meant that all I do on a trip basically is buy a one day Disney ticket to one of their parks. The savings incentive for a second or third day at a Disney is just not big enough for me to go for it (sway me to their parks vs. picking whatever two or three parks in the area we feel like doing too), so I tend to do a Disney day or two and a competitor park too (usually Sea World).
On my December trip, for example, I will buy a one day ticket for SeaWorld, a one day park hopper for Universal/IOA (don't usually go to that one - prefer Disney and SeaWorld, but decided since for once I have a trip low season I'll go see the new HP stuff and that requires a park hopper to see all of it / nice to see a few new things), and a one day ticket for the Magic Kingdom. As a one or two Disney day person, I do vary which I choose on a trip and have done DHS, Epcot, and AK before as my only park. Recently, though, DS has been picking the MK most often. There parks are very nice and for a once a year expenditure, I think $100 is expensive, but not out of line with what is offered.