Clean install of ML really necessary?

It's not necessary, but some have claimed that it resolved problems they encountered after updating. It won't hurt to update first, then only perform a clean install if you encounter any problems. It appears those with problems with a regular update are very much in the minority.

 

I usually perform a clean install, but this time around I did all three Macs as upgrades..No issues or problems to report, a very quick hassle free process. A few Apps had to go, but hat's normal.

 

No, a clean install is not needed, therefore listing pros and cons isn't needed either. There are plenty of threads with supposed pros and cons if you want to waste your time reading them.