In my opinion there is big misunderstanding what a BLIND exchange is for. It seems that some join the 100 club with expectations to top up their collections with missing stamps adding impossible requirements that cannot be fulfilled by many other members (e.g. limiting countries, high values, large commemorative etc). I don’t understand what large(?) commemorative means. In Japan most pretty current stamps are commemorative but not large. Different catalogues may define definitive/commemorative differently. So, is it fair to downrate any unfortunate sender who pulled out your name out of limited numbers of members but cannot meet requirements from your profile simply because he/she doesn’t have what is asked? I think that any respective collector whether amateur, beginner or advanced would try to meet individual requirements of the person if she/he can, I usually do. However, every serious collector knows that situation with postal stamps has changed in the modern IT world. It is not easy to get used stamps in general, unless start using auctions, e-bay etc and many lots passing through dealers hands anyway.
So blind exchange should be treated as a surprise - it may be pleasant and you can add few stamps to your collection or may not be on this occasion. However, if you received 100 stamps and it is within the club rules (no damaged stamps, used stumps, no doubles etc) then there is no ground to give a sender low scores.
I’d like to comment on how post office staff work based on my experience in the U.K. where only definitive stamps are available at the post office counter. This is because most of PO are now located in small private shops and perform a lot of other services like checking passport applications, processing different payments and many others things in addition to sending parcel, special delivery letters etc. If you need commemorative stamps you have to buy them on e-bay or order on-line from Royal Mail. According to current rules (as I’ve been told) they cannot even cancel stamps. You buy a definitive or Christmas stamps, stick it to a letter and drop it the post box. Stumps all cancelled centrally by machine hence a “lovely” wavy lines on many used stamps.