While I understand where you're coming from (and am a minimalist park-goer by myself), it wouldn't have worked when my DD was little, or when we start taking my grandkids.... While there's definitely inconvenience in carrying more things into the park, it's not just "a few dollars" I'm saving... For instance: (prices approximate)
12 pack of water bottles at 7-11: $7
1 bottle of water in park: $3.25
Savings: $32 per 12-pack (times this by the 3 days we are in the park, and I've saved $96 by packing my family's water); tap water at DLR isn't an option for us, or we could be saving much more!
1 locker to keep sweatshirts in: $7
1 sweatshirt for me in park: $60
1 sweatshirt for DH in park: $60
1 sweatshirt for DD in park: $40
Savings: $153
3 snack-size lunchables from 7-11: $6
3 snacks in park: $15
Savings: $9
On a trip with our grandsons, a small diaper bag with 3 diapers and a change of clothes for baby (and a shirt for me) means that we would probably head to the hotel at least twice a day because I'm covered in baby puke, or the diaper exploded, or our relatively newly potty-trained little guy waits a little too long. If we say 2 trips back to the hotel, that's approximately 60-90 minutes we've spent not enjoying Disneyland, just because I didn't want to pack a larger diaper bag (to have in the locker that I "purchased" in the above example). I'd much rather spend $7 to throw some things in a locker than heading back through the whole park, out the exit gates, through the esplanade, through the pedestrian entrance, across the street, down the length of our hotel (at Park Vue), unload 3 kids from the stroller, get 5 people through the bathroom, a baby (and/or me, and/or toddler) changed, disconnect 3 kids from their toys that they managed to find while everyone potties, pack everyone back into the stroller, lock up the hotel room, walk the length of the hotel, wait for the light, cross the street, walk down the entrance, through bag check where I have to unpack the stroller to find the minimalist diaper bag, then find the tickets to get into Disneyland, wait in line, get through the gates, and then try to remember where we were (or agree on where to go).... Holy cow, I'm tired just thinking about it!

Props to parents who go minimalist in the parks!!
I think it just depends on what is important to any specific family. And, as with everything, one size doesn't fit all.