A. It's simply a matter of preference. Rake doesn't give you the much of an option. Ergo does.
In your .ergo/script.rb file just put:
import 'Rulefile'
And then you can put all your rules in the Rulefile instead. Or if you pefer, make a rules directory
and use:
import 'rules/*.rb'
How you go about it is up to you.
A. There are a couple of reasons. The most important of which is that Ergo states can depend on more than just file changes. State's can be defined for any condition whatsoever. This make Ergo quite a bit more flexible than other tools in it's class. The other reason is the file monitors are not cross-platform. This adds additional complexity to dependency and installation. Obviously, in some ways file monitoring is nice --it's a bit more responsive and reduces runtime overhead. But it isn't in any way vital. Ergo trades those niceties for greater flexibility and ease of deployment.