Why is X-Perl so popular?

[wowunderachiever]

I have been curious about this a long time. Across the various add-on sites, X-perl seems to be head and shoulders above all other unit frame add ons in popularity.

Why is this? I'm assuming it is because it does something better, but what exactly? Does it do things that are hard to find in other add ons?

I haven't been able to change the looks of it enough to suit me. My only complaint with it has been cosmetic but I wonder if I am missing out on some important functionality by not using it?