Because it's a crutch. It's relying on development in and of a competitor's ecosystem. What incentive would a developer have to write a native app when emulation is available? Without native apps a free OS can't generate revenue.
I can think of a lot of reasons why this would benefit ubuntu phone. It would entice more people used to the android platform to try yours. It would also give people a reason to go from a more popular platform they used before to another. Old linux interfaces used the start menu from windows for years and still complain when distros take it away. Bluestacks allows windows users to play android apps but Bluestacks doesn't seem to threaten mac or windows platforms. It just gives them a reason to go back to windows and mac to use their android apps. Many apps on android are free but the free apps keep people going back until they do buy something. There's nothing stopping Ubuntu from selling their apps on the android store. Getting people more of a reason to try your free platform and use it gives your platform a chance to sell something.
+Brian Perry it does and for the moment being they will use android apps until they replace it by their own native apps. I can see myself testing it. A sailfish os with all the popular apps in it. ( reason why I didn't got WP8 yet) so that's a kick starter for sailfish compared to Firefox and ubuntu