Grunt has a core file selection abstraction. Adding mtime source/destination checks and removing files from the list as needed should let them add this without issue. When I looked at the file internals for a bug I encountered I didn't see why it wasn't added but I was/am in the middle of a different project so I didn't take the time to figure out why.
The lack is not a great reason to write yet another build system.
I'd agree. It's more like a composable set of shell scripts at the moment. How things fit together differs quite a bit from other build systems I've used (make, rake, ant) and after a significant amount of cursing, I've come around to it.