You may get a better seat. But it depends on exactly which airline. Mrs radeng flew a Qantas/BA code share to Sindapore today: she couldn't check in on-line on the BA website. Even the BA Exec Club in Munich (where she was yesterday) couldn't help, and despite being there before the check in desk opened this morning, she couldn't get an aisle seat when it did.
Which naturally led to her telephoning from Heathrow, waking me up, so she could bitch at me about it -as if either it was my fault or I could do anything about it!