Wow...this kind of levels the playing field. A big reason I stayed away from IE was because of the extensions. If people's favourite extensions start working on Edge, then I'm sure many will give it a serious go.
Edge will have to prove itself as a browser first. IE is inferior to all the others, currently. Edge, as "Spartan", has shown positive results in Microsoft's "lab tests" but they showed same results in the past so viewing such "reports" should be taken with a grain of salt.
If they can make this work seamlessly, its a great way to steal users from both the Chrome and Firefox camps. Single-browser extensions are in effect "bricks" for building a walled garden around a browser.
So I'm happy if MS is able to break down those walls and take us more toward an open browser standard, as yefim commented.
Many reasons. Leaky abstractions, possible slight incompatibilities/quirks due to having to maintain two extension layers at once. Generally development times split between two completely different environments.