Not always. We don't know what happens to xbar between

foo: bar = $(xbar)

and the point where bar is expanded (e.g., command script). For example it can be set to some other value.

Also it becomes quite a headache to maintain things like this:

foo: bar = $(call xbar,$1,$2,$3)

Now I need to remember to change bar definition every time I add/remove an argument in xbar.

Make does not support defining target/pattern-specific variables using define/endef:

foo: define bar
...
endef

This makes it hard to define multi-line target/pattern-specific variables. I realize that adding this feature will break makefiles
that happened to have a rule with the first target being 'define'. We could use a different syntax:

define foo: bar
...
endef