I disagree. Let me explain...
Right now, the only date-sensitive ticket pricing that
Disneyland really has is for the 1-day tickets. The prices on the multi-day tickets are the same regardless of which days you are at the parks. But when you buy a 1-day ticket from Disneyland, the price varies if you are going, for example, mid-week in January compared to on July 4th. Just like the 1-day ticket prices now vary at Universal Studios Hollywood.
With aRes travel's DL tickets, though, you CAN encounter a price difference on park hoppers if your travel dates change. I ran into this problem last October and aRes doesn't really mention it on their website. If you have to change your dates of your trip, then you have to call aRes and they have to issue you a new ticket voucher. If when you call them to change your dates, there is a price difference on the park hoppers, then you have to pay the increased price. They will not honor the prior sale price.
Buyer beware