Well, it depends.

At iDine, other than BA (and even in terms of bonus promos there), yes.

At rental cars, not necessarily: While ALL rental car places seem to give a platry 50 miles per day with US carries, with some overseas carriers you can do much better. For example, Avis gives 500 BA miles for even a one-day rental.

And with some hotels, it can be a big difference too. At Priority Club's Candlewood Suites, where a room might be say $85 a night, you only earn 1 mile/$ with US airlines, and thus a platry 85 miles for a one-night stay. But with BA (or a number of other European airlnes) you earn 500 miles no matter what the length of stay!

IF you have a Diner's Club card, the seemingly annual 50% transfer bonus to BA only (once upon a time it was even 100%!) also is unmatched by any US program (on the transfer side).

Thus it can make sense to have a "secondary" European airline you collect with "slowly" by non-flying, for those opportunities where you can't earn anything worthwhile with any domestic airline.

And, ironically, those are often airlines you DON'T want to earn by flying. If you fly advanced-booking economy class, you earn only 25% of actual miles flown over at BA. Yuccch! That means I want to credit all my BA intra-Europe flights to AA, and ONLY earn BA by NON-flying!