I try to book all AC metal/codeshares quite a bit for my PTY - Europe flights (through YYZ). The main reason for this is miles for Copa/ConnectMiles. Whenever I book an N fare from PTY, it books into P/D/Z from PTY-YYZ and then N from YYZ-X. Since I usually do 4 flights each way (for miles) the 2 intra European flights are normally on LH or LX. If I was to book without the codeshare, then it will book into Q (or so) with LH. This gets me 50% miles. However If its all booked with AC metal/codeshares, then it remains an N fare and will book into B on LH, which in turn gets me 150% miles

The difference in miles is around 1,300 per roundtrip, so it all helps for status:

PTY-LHR (via YYZ, MUC/ZRH, FRA) on AC AC LH LH (all AC metal/codeshares) on an AC N fare would net me around 9,500 status & redeemable miles
PTY-LHR (via YYZ, MUC/ZRH, FRA) on AC AC LH LH (but not AC codeshare) on an AC N fare would net me around 8.850 status & redeemable miles
PTY-LHR (via YYZ) on AC AC on an AC N fare would net me 8015 status & redeemable miles

So I definitely disagree with the statement of there being no upside for customers. There are definite downsides though, as noted above by others