Agreed, but only to a certain extent. A mug with an uninteresting animal picture, or placemats with the zoo's logo are nothing for me, and neither are the same plushes or animal figures you can get in all zoo gift shops, but some zoos really do try to offer some more interesting things...I don't buy anything in zoo gift shops because I'm not 12 years old.
Leipzig's gift shop had books with overviews (going 5 years in the past) of a year worth of construction and animal news, complete with full list of all species on-show (I think Plzen does this as well). Antwerp sells postal cards and posters with the original artwork from the 1930's. Artis sells original historic animal signage (though unfortunately at very high prices). Burgers' Zoo and Apenheul both sell a lot of the important and well-known 'zoo books'. Paris Vincennes had a lot of architectural books, describing everything from the old zoo to the new zoo and everything in between, complete with concept sketches etc. I don't often buy things, but I always take a look in gift shops just in case there is a hidden gem lying around...
About the sandwiches, I always plan to take those with me but always forget to, so I usually just end up eating in a zoo cafe anyways (or buying something right before I enter, which is what I often do in Antwerp).