The only problem I see is with MQMs
All code share flights do not give full MQMs.
Particularly - with Delta coded flights operated by Continental!
On Delta-coded Flights Operated by Continental Airlines, for L, U, T fare classes, you will only get MQMs = 0.5*Miles (minimum 0.5*500 = 250),
while the same LUT fare on "pure" Delta flight would have gotten you 100% of MQMs.
Also, "M" on pure delta gets you 150% MQMs while "M" on Delta-coded NW/CO gets you only 100% MQMs.