- They're not all about coding standards: only two of them are. PSR-0 is a class autoloading standard and PSR-3 is a common interface for loggers. The next PSR is likely going to be a common caching interface.
- PSR-1 and PSR-2 are controversial (although PSR-1 less so than PSR-2), and don't necessarily represent best practices amongst the larger PHP community. They're useful insofar as the PHP Framework Interoperability Group (PHP-FIG, http://www.php-fig.org/), the people crafting the PSRs, needed an internal standard and really shouldn't be represented as anything more than that. Much of both standards are an amalgam of individual practices produced via an internal survey of PHP-FIG member projects.
If you have nothing, PSR-1 and PSR-2 aren't bad places to start, but diverging from them is more likely to happen than not, especially if you're working with projects or teams that have existing coding standards.