I hope someone can correct me if i overstep my intelligence...
You're right about being able to connect a MIDI interface to a joystick port. This could be all you need if you don't plan on doing heavy MIDI work (if you're only using a keyboard to play a soft synth). There are 2 issues though. 1) I don't know if you can plug in multiple adapters on top of each other (for some reason, this sounds like it wouldn't work), so you may be limited to just one cable of I/O (16 channels). 2)
http://www.midi.org/about-midi/abtmidi.htm says that ground loops are potential issues for these types of adapters. What that means for your MIDI data, I don't know. It doesn't sound good for your audio though.
Getting a pricier interface won't affect the sounds since the waves don't come from the interface. It will give you more routing options with more I/O, and some will boast lower latencies. Higher end audio packages can lack MIDI if they're dedicated to audio (didn't Pro Tools just recently integrate MIDI?). For instance, if I'm recording my band in my basement, all I really need is to get the audio into the computer. I don't need to involve MIDI at all.
I can't attest to the bugginess of USB stuff. It sucks if you've been having problems. I can say that I've not once hit a snag using
the Midisport 2x2.
As for MIDI thru ports, they're a great convenience, but if you start getting more than a couple MIDI things, you'll realize that routing can become intricate depending on how you're controlling your setup. I only use three MIDI-fied pieces of gear, and I have to constantly replug the cabling depending on what I want to do. Latency can also become an issue if you're daisychaining multiple pieces together. That's why I will need a patchbay, especially if I want to expand my gear in the future (you *will* want to expand your gear in the future).
That's about all I know about MIDI (I just got my first cable a couple months ago). I hope someone corrects me if I said too much.
-marcus