Much like a few other manufacturers, Sony Ericsson likes to keep thier hardware proprietary. Basically, so that they can make more profit. Just like its memory card format. Most other manufacturers use MicroSD, while SE uses the M2 format.
No, all SEs either use the older port or the current fast port. Some phones use 2.5mm, and sometimes are equipped an adapter. The adapter is there if the user would rather use a headset of higher quality, and one that would be prefered.
I didn't notice the jack until after I bought it and thought it was stupid too. I should have remembered that Sony does that after I bought an mp3 players years back and had the same issue.