Should't juniors be afraid of gaining experience in a mediocre field (to them; say web development) then not being able to switch to another field?
plus you have to take initiative to learn and grown, your boss or who ever you work under does not have time to sit with you ever day and guide you like a counselor. he has errands to run every day.
and about not being able to switch, so all you need to switch to another field is a reliving letter and an experience letter. and you can switch. i went to work for a bank for a while after my first failed startup. there will be no problem.