I'm not read up on either lirc or IR in general so I might be completely of.
pronto codes seems to be just a formating and/or standard on how IR-signals shall be sent. A long big hexnumber wich probably says how the signal should look like to give a specific IR signal.
This seems to be used in universal remotes as then one only need to know the pronto code for the old remotes key to have the universal remote mimic the old one.
So i think the reason for why USB-UIRT needs this is because it maybe uses it to send the signal forward to the next.
All in all I can't see an advantage for XBMC to have pronto, maybe if one would like to control the vcr through XBMC (Then the user could just tell the pronto code for REC etc. in XBMC)
I think a device using pronto code just as well could try to mimic the MCE remote instead and it would work with XBMC.
But as said. I have only googled this for a few minutes so my grip on the subject may be flawed
On the topic with USB-UIRT:
Taken from mythtv wiki :
http://www.mythtv.org/wiki/index.php/USB-UIRT
Quote:The USB UIRT allows any USB-equipped PC to transmit and receive infrared signals. LIRC 0.8.1 supports the USB-UIRT natively. LIRC 0.7.1 does not support the USB-UIRT natively, but the patch listed below can be used.
On ubuntu gutsy lirc is 0.8.2 so from what I can tell it shouldn't be difficult to get the device running without changing anything in the code. It probably asks at install time for USB-UIRT?