I think it depends on the purpose of your trip. If you think this is going to be "the trip" that you ever take then I think you should cut back to two cities at seven nights each. If you see yourself back in Asia in 2007 or 2008, then I think it's fine to do three cities.

What's your long term strategy? How does this trip fit in with other planned trips for the next 4-5 years?

I find that I my schedule works better with 4-day domestic trips, 5-day and 9-day trips to Europe, and going back to places multiple times. But in Europe I was fortunate to have a base of a 37-day and a 21-day trip back in the early 90s when I had an employer who was generous about vacation time to make up for the low pay. So my normal trip to Europe these days is a trip that includes places I've been before plus one new place. If I like that new place, I might make it the old place on the next trip.

With that strategy in mind, I have a trip similar to yours for my first Asian trip, planned for next April: 13 days across SIN/HKG/KIX/NRT. I know I'm shortchanging all four of these cities, but they are all candiates for repeat stopovers on either 9-day regional trips (Japan/Korea, HKG/Macau/Taipai/etc, SIN/Malaysia/Indonesia) or 23-day RTW/CirclePacifics to Australia/New Zealand. The goal of the trip is to see if I even like the culture in the first place, and then to start prioritizing locations and setting up logistics for future trips.