Tricky one this... I've had it on a handful of occasions.
Personally - although I try to talk couples out of it, I do let couples dance together if THEY REALLY WANT TO but what I say is to step back slightly from the circle so that "partner passing" can go on without collisions.
My reasons...
1) They are paying customers and when it comes to cold hard cash - they're paying me to teach them so if they'll leave if seperated, I'd rather keep them together than loose them both.
2) When I went to My very first salsa lesson, I went with my girlfriend. Danced together for 30 seconds, then never saw her again until the end of the class. That really frustrated me so I know how couples feel.
3) You can improve quicker with a regular partner so it makes sense to dance together.
Saying all this - I do encourage couples to seperate because...
a) They get do understand different leading / following styles.
b) Singles get to dance with a partner
c) No one gets stuck with a "muppet" for too long
d) A relationship couple will have a "domestic" there and then if one thinks the other is wrong. Normal couples don't argue with each other (usually)
At the end of the day - I do not feel a teacher should push the issue if a couple want to stay together but should point out the advantages of swapping. I never usually find it a problem and would only find it awkward if there were 10 couples who won't swap and one spare lady!
By the way - don't forget to come to David Lloyd on 5th June. There'll be plenty of partner swapping. Hee Hee Hee (Shamless plug)