This is a known discussion in the PHP community, you can google about it:
http://stackoverflow.com/questions/4690478/functions-vs-static-methods

The plugin options API are called many times, and this is not the case!
Every micro-second counts so let's try to keep the plugin fast as possible :)

I love the OOP style, but we shouldn't be so selfish to use it everywhere, there are some places that make sense to use the old-style programming and some not!