Join Date: Apr 2002
Location: Montréal (Québec)
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Good question. Derailleur braze-ons seem to be found at both extremities of the spectrum. On low-end bikes, this is sometimes done for faster assembly, whereas on high-end bikes, it is "more pretty".
There are a few drawbacks, however.
1. As you have discovered, you can't adjust the derailleur height.
If you change the rings for something really larger (ex. 50-40-30) and are screwed, you could file off a bit of the derailleur cage, but there are obvious limitations and you can't come back.
If you change the rings significantly, there is another issue, especially with STI: derailleur-cage curvature.
2. Some claim that the seattube is weaker and may break prematurely. I can't remember if it is Rivendell, Heron or Waterford that claims that on their website.
When I compare my old and newer bikes, I would suggest that derailleur clearance isn't as critical as it used to be, and it's quite likely that it would not work better with 1-2 mm clearance than with 6-8 mm clearance.