This is at least my personal view on things after reading what Google themselves say about ART.
ART? what is ART?
Is the the new experimental runtime Google introduced in 4.4, Experimental is the word here since it’s not ready to replace Dalvik just yet. Perhaps in the next major version of Android it will.
This is said by Google here
Dalvik must remain the default runtime or you risk breaking your Android implementations and third-party applications.
I don’t that HTC is so keen on including something that’s experimental and that might lead to erratic behaviour in apps or that apps simply stops working (like whatsapp etc).