Because, while such modified cassettes may work with some true disc wheels (but not covered ones like those by hed or flo), they won't work with many spoked wheels. The issue is not the RD and the spoke clearance when shifting to the biggest cog. The bigger problem is that when the back of an 11-speed cassette is milled by ~1.5mm, the clearance between the back of the cassette cogs/cassette spider and the hub body/hub flanges becomes extremely tight on many wheels. Sometimes the clearance is zero or is a negative number. From measurements that I have taken, milling the back of a cassette won't safely work on a 2006-2008 zipp 808 wheelset for these reasons. So I did this instead, simple and keeps everything future-compatible too.
Greg @ dsw
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