I don't see how you can get points transferred into your account without giving out your member number. I've read that the transfers are often done with both members on the line with MS. Maybe that process would alleviate your concern.
If you are worried about security (the other owner accessing your account later without your permission), I think you need more than a member number to get MS to execute a transaction. I always have to give them the lat 4 digits of my SSN as a verification that I am the owner of my account. I don't see a reason why you would have to give out your SSN to receive points. JMHO. Maybe others will have a different take.