JerseyDad, IMO letters are fun things to get and it's better to receive them than to not. I don't think that mail indicates any kind of serious interest though in a player from MOST schools. I do think that the mail provides players an opportunity to pick up the phone and make contact for perhaps the first time.
If a school has called and then is sending mail in addition, that is much more indicative of true interest. However, the only way to gage interest is if an offer comes along.
When my son was in HS, he received letters (frequent letters) from many large and smaller schools. I'll use Boston Colelge as one that he received mail from typically a couple times a week for months! I always felt like the coach there probably didn't even know his name! He was strictly on a mailing list IMO. Different programs use mail for a variety of purposes and it's not always because they're seriously interested in that player as a recruit. I am continuing to this day to find letters and media guides stuffed in various places (two years later).... from schools he never had a call from.
Again, I would suggest to a player that he always return questionnaires. However, if you receive mail from a school you're highly interested in, pick up the phone and call. Then see if the coach continues to make personal contact with you. JMO