Any disadvantage of using .shtml ?

I am trying to include a footer on every page. So I made a footer.htm
Now, I am including it on say, main.htm as follows:

<!--#include virtual="footer.htm"-->
</body>

For this to work, do I have to save main.htm as main.shtml?
Are there any disadvantages of using .shtml files, like compatibility issues, performance problems, etc?

Thanks