I have often experienced that by pricing an itineary with codeshares was alot cheaper than using true flight nummbers speeking mostly of UA/LH codeshares. Maybe there is no big difference in domestic fares but it did a huge difference on my international ticket. aswell i think that i recieved more bonus miles on the codeshare than on the LH flight on the same route. Its wright that the airlines cannot provide language service on each codeshare flight but i had the experience last month on a UA flight out of London that there was a Swedish speeking flight attendant while chating with her she told me that the company tries to have at least one scandinavian speeking attendant as they codeshare this flight with SK.
Another thing i like with the codeshare flights is that UA can actualy request seats
for me on them this saves me time as i dont need to call the operating airline again.
I started liking the codeshare system some time ago before i got thet senator advantages. There where no more seats on a LH flight i realy needed and my travel agent found seats on exactly the same flight on the UA codeshare number.This saved my day since then i often chek if there is a better availability on the codeshare.