I think this is a pretty impressive way to get the characters into the parks in a pretty non-obtrusive way.
First of all, from a Disney standpoint - almost no expense. Paying the voice talent for, what, maybe an hour of work. Paying a tech to reprogram the lights for, what, maybe 15 minutes of work. Not that I think it will happen, but, in the event that P+F suddenly wanes in popularity - they have an attraction (not a ride or really a show, but technically an attraction) that cost next to nothing. And it satifies (temporarily) some of the folks who have been clamouring for P+F in the parks.
Second of all, from a Disney family standpoint. Does our family love P+F? Yup. Will our DD go nuts when this happens as we walk the streets of America. Yes. Will she want to stay an extra hour to hear it happen again? Yes. (which, back to the Disney standpoint means we are buying a snack or two and possibly visiting some shops during that hour - although, I personally will just go see MuppetVision 3 times in a row).
Third of all, from a Disney nube standpoint. I pointed out the video to someone at my work today and she watched it. She has no idea who P+F are and has never been to Disney. She loved it. Thought the plot was hysterical. She could immediately identify the character of Doofenshmirtz pretty accurately. She was a bit confused by Perry the Platypus but got a giggle out of the word platypus by itself.
Fourth of all, from a theming standpoint. Doesn't Doofenshmirtz try to take over the tri-state area every episode? So, wouldn't taking over the 'Streets of America' fit this conceit pretty well?
I look forward to seeing the lights this weekend and I am certain we will make sure to catch the Phineas and Ferb portion of the show!