While I obviously can't speak for everyone, I can attempt to speak to why I, personally, write stuff with Tk:

1. Documentation. This is a biggie. With two books out on Perl/Tk (the aforementioned Mastering Perl/Tk, as well as Learning Perl/Tk) it's fairly comprehensively discussed in dead tree form -- if that's how you like your documentation. It also has much more comprehensive digital documentation, as well; I mean, it has about a hundred pieces of POD, including one for every widget.
2. Portability. Despite what you say above, I was unable to find reference to Gtk working under Win32, or qt. In either case, it looks to be a hassle, and the few places I did see reference to it, people sad "if it exists, it's probably buggy." I'd be inclined to agree with them. Tk installs well and easily under Activestate, as one can just use PPM. I saw no such option for Gtk or qt.
3. User base. From what I've seen, there are quite a few more people already using Tk than use Gtk or qt. This means that it's easier to find people with clue when you get stuck. Unfortunatly, this is a self-enfocing problem.
4. I don't care that much about the ugliness. For people who care about that extra bit of spit and polish, perhaps Tk is a cancer on their screen. That's probably the price of being cross-platform, though.

perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'

Quite apart from real cross-platform capability (rather than "it works on Linux and you might just about get it to work on Windows") Tk can look good if a bit of thought is put into the design.

Another Tk tip is to make use of the standard dialogs, which should use the dialogs provided by your OS (if any). For example on Windows you can use the standard file open and colour picker dialogs.

Have a look at perltk.org as well, lots of examples are available there.

Kevin O'Rourke