MPLv2 is a fine weak copyleft license, but I'm not really sure it prevents people from not contributing back if that's what they want: just put your proprietary code in a different file, add minimal API's to the MPL code so you can use it.
So in that sense it's only very marginally 'better' than a fully permissive license.
Enter MPLv2, with the best of both worlds:
- not viral copyleft
- but users DO have to contribute back