Cost/benefit of interim professorship
The answer will depend on many individual factors. Some positive aspects are:


The university gets to know you. I know of several interim professors who got a full professorship later on because everyone was satisfied with the person. 
You are gaining teaching experience. This is an important factor if you want to apply for a professorship.
Related, but slightly different: You build up a teaching portfolio. If you are becoming a professor later, you are already having some material at hand.
You are getting in contact with motivated students which you can involve in your research activities.
Last but not least: Teaching can be fun :-).