Pros and cons of supervising master's students as a postdoc
If you are planning a career in Academia that includes any non-trivial teaching component, it would probably do you good to get some experience with this. Whether it is enough to compensate for the time is a judgement call. 

But, one thing that you need to learn as a teacher is that other people don't learn like you do. Every student is different and each has a unique way of learning. Things that are easy for you may not be for others. This is more pronounced at the undergraduate level, but even at the MS level you will get a sense of the kinds of struggles that students have by doing some supervision. 

The experience may also give you some insight into coming up with the sort of problems that are appropriate for MS students. Problems that are hard enough so that something can be learned but not so hard as to require years of effort. Having a lot of problems at hand can serve your own research interests well. 

A more remote advantage is that some of these students will continue to a doctorate. When they do, they will form part of your academic circle with you as a "senior" member. This can open possibilities for collaboration later. 

I think there are a lot of pros for this and few cons as long as you don't take on too much initially. Since there is only one such offer currently, it might be worth it to dip your toe in that stream.