Is it wise to do teaching-assistantship in a university I am not studying in?
Benefits:


You keep your contacts with the academic environment, that may be useful applying for a PhD.
Some professors will know you, and this that may help you get a position (depending on how it is done in your country).
You gain teaching experience.


Drawbacks:


TA salaries are usually quite low. Depending on your background, you may be loosing money. The working conditions may also not be so good.
You are not getting other kind of experience, that would be useful for any non-teaching job.


On a side note, it may look weird on your CV being a TA while not enrolled. This is not necessarily bad, but be prepared to be questioned about it in interviews.