Backwards compatibility is extremely important--existing APKs will still work with ART. They also avoid separate binaries for each supported architecture, MIPS,

Backwards compatibility is extremely important--existing APKs will still work with ART. They also avoid separate binaries for each supported architecture, MIPS, x86, and ARM. Plus this system also allows apps to take advantage of updates to the compiler without requiring that developers recompile and resubmit. Imagine in 2 years a new ARM version drastically increases performance by using some clever new instructions. If you have pure native code every developer has to recompile their apps to take advantage of the new architecture--with ART, Google handles the update, and every app on the appstore benefits automatically.