Yes, as mentioned, NOW you can take them between the parks. Back in '05, according to my rental ticket, you could NOT take them between parks, and had to bring your ticket to the other park to get your other ticket. But now it's different.
The length-of-stay deal is NOT a money-saving thing, but a time-saving option. You pay up front, and they give you the number of stroller tickets you'll need. Each day, instead of waiting in that long line, you just go right up to the person doling out the strollers, almost like you're going in the exit of the line, hand over that day's ticket and receipt, and boom, you're given a stroller. If you save your name-tag from day to day, that's even less time it'll take...the stroller lady was amazed that I'd kept mine from day to day, and that I smiled, was not in a hurry, and said kind things to her...guess it was a bad morning, only a few minutes after opening!
I'm personally, now, a fan of bringing my own...I don't have a Disney Visa and have absolutely no plans to apply for one...we leave nothing in the stroller and we turn ours around when we park it so the handles are awkward for a sneak thief to snatch it (we haven't had the experience of CMs moving things around as we always park it in an extremely proper place AND we go at quieter times), and we try to not worry about it. It's always the walk back to the hotel that's the killer...that and trying to get DS to hurry to the parks (he'll try to run once IN the parks, but wants to dawdle on the way there) in the morning...so those are the biggest Must Have Stroller times for us, and you just don't get that if you only rent a stroller.