The default Blend mode of strips in the VSE appears to be Cross. While there are a great many use cases where this is not a problem, there are a significant number where Alpha Over is a better choice. Add to this that Alpha Over is also appropriate in most of the other cases.
I predict that this change will reduce the number of users who are confused when they try to do overlays using the VSE, and reduce wasted time as users hunt down a solution.
Therefore I think that making Alpha Over the default Blend mode for VSE strips is worthwhile.