I agree--it would probably be great experience.
One thing almost all nursing students seem to have to get past is the sense of ill-at-ease when first taking care of patients. Being an aide gives you lots of opportunity to help in ways that are important, but not life threatening, if that makes sense.
You can develop your sense of relating to patients in beds, and get comfortable with hands-on (literally!) caring. The only impediment I can see is if the hours are such that it interferes with studying, or if the pay or situation is so poor it is depressing or overwhelming.
Have at it--all an extern is, really, is an aide who has been to at least a semester of nursing clinicals, and who gets paid a little mroe (usually).
Nothing is ever lost from an opportunity taken. Nothing is ever gained from an opportunity missed. (that's an original....