Unless you're just daft and just want to disagree, it's pretty common knowledge that 3G/LTE drains significantly more battery than EDGE/GPRS (lots of people drop to Edge to save battery), and we all know that WiFi data consumes less battery than 3G/LTE data transfers.
That's where the big difference is. Saying the WiFi is going to be on all the time doesn't matter all that much, becuase the battery drain of the WiFi radio is not that high to begin with. Most of the battery drain in a smartphone comes from Data Transfers, the Screen, and Location Services. The fact that WiFi has much less battery drain for data transfers is what makes it superior, coupled with the fact that you get faster location triangulations when you use WiFi in conjunction with the GPS chip.
If you look at any specification breakdown, talk times on GPRS/EDGE are almost always higher than on UMTS for the same reasons, and when a phone is on WiFi the device completely avoids 3G for data and uses WiFi (which uses less battery) instead. When a device is used for streaming media (Video, high bitrate audio) WiFi is clearly superior to 3G, and also generates much less heat.
The reason why Microsoft did it? Ask Microsoft. They do rather narrow focus groups with relatively weak justifications for a lot of the changes they make in their software. I'm not here to defend what they did. I just make a choice on what products I move forward with in the future.