Let's compare:

$window->title('foo')->border(20);
# versus
$window->set(title => 'foo', border => 20);
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Clearly, I like method chaining. I use it frequently and my primary argument to people is also "don't use it if you don't want it." However I would never encourage method chaining if the mutator didn't return an object of the same class as the object that received the message (or maybe with an isa relationship and with constructors being a special case). That's just begging Demeter to beat you to death. However, when I look at your code, I see one great big "catch-all" mutator where now I no longer can merely validate the values, but I also have to validate the names of the attributes. It would be easy to type this:

Depending on how someone codes that, it could easily silently fail when a direct method call would not.

I suppose we could argue all day long about which technique is better, but both of our preferred styles have strengths and weaknesses. For now I'll continue to return $self and you can continue to ignore it :)