I'm aware that adding an object puts a dependency on PHP 5+.
Basically, the primary reason that I want to use an object is beause objects maintain their own state. The HTML filter in Drupal core (which I borrowed code from, actually) maintains it's own state through a static variable for compatibility with PHP 4; but the solution is a bit hackish. Plus, there's a lot more context that needs to be stored with this filterer: things would be a lot more complex. Additionally, I want to eventually implement caching... caching an object is as easy as serializing it.
Besides, Druapl 7, which is supposed to come out fairly soon, is going to require PHP 5... so if you haven't upgraded to PHP 5 yet, please consider doing it!