However, I don't think any park will make the discount big enough, say 50% off, to have any noticeable change in crowd sizes. You are already paying many times the ticket price on the other associated vacation costs and a $10 to $15 a day discount won't be an influence on vacationers.
We accept variable pricing in airline tickets, in train tickets, in retail goods, (ever tried buying a new kitchen in the month before the January sales?), so I can't really see that variable pricing at theme parks is really such a great step. I think it's inevitable.
I fear that if the prices are variable throughout the week and year that no one will be able to track any sort of distinct price increase. If the prices were set by quarter, by day and were set at least three months in advance, I would think that was fair. But if prices can change on the fly like Airline Tickets or gas prices I'm not an advocate of that type of price structure.
On a second note the AP's (at least at Disney) are already based this way with the lower cost AP's not for use on the busier days.
Kings Dominion offered cheap tickets for passholders on fathers and mothers day, and it turned out that the park was a lot less crowded for a sunday than I would have expected, so I guess that was a good deal for them.
And their halloween is priced lower on friday which is only open 5 hours, than saturday or sunday.
Perks are also priced more on bigger days. The "front of the line" passes cost more during peak times than other times.
Really, the main people this might encourage to reconsider what days they visit on would probably be the locals. If you live in an area and know that it will be cheaper to go on certain days you might adjust what day you are going to accommadate that.