I'm currently extending my house in the UK (where this construction is less common for full houses) and the upstairs dormer is built in much the same way. While I wouldn't have described it as rickety before sheathing, it's definitely much more solid once the walls have each side covered in well nailed-on OSB - it gains a lot of stability that's otherwise lacking to a degree as there's no diagonal bracing (other than some temporary elements which are removed as you add strength).

I'm not a builder (I'm a musician for the most part), but what I've built is to a pro standard and millimetre-accurate. That wouldn't have been the case had it been post-and-beam!