Knowledge is power.
Coming to the question of serving the guest in a proper manner, I believe if the thing given in snacks to the guest is things like Namkeen, Biscuits, fritters or things that are in individual pieces then it is ok to provide the snacks in a single plate if the things given in snacks that do not have individual pieces such as sweet like Gajar ka halwa than it should be served individually to each guest in different plate, as not everyone likes to eat from the same bowl.
"It is hardest thing in the world to be good thinker without being a good self examiner"
Coming to the topic, I fail to understand why the author should feel bad about being served in a common plate. We visit others when we have that bonding and so where is the question of having separate plate to serve you or anyone else? As Neelam has rightly pointed out, the servings would depend upon the type of snack that is being served. For example, if you are serving Samosa with some chutney or sauce, you may keep the samosas in one serving plate and give quarter plates for the guests to take what they want. The serving would be different with namkeens or other such items. So, it depends.
I don't think there is anything to feel offended about being served in a common plate. After all, that is bonhomie!
"In three words I can sum up everything I've learned about life: it goes on." -Robert Frost
In Summer we will get good mangoes which are to be cut and pieces are to be consumed. My wife will cut 4 or 5 such mangoes after removing the skin. She used to keep those pieces on the table and while discussing we consume them. When guests are therewith is also we will continue the same practice and I never felt anything wrong with this practice.
When we attend meetings also, biscuits will be kept in the middle of the table on a big plate and people can take as they like and eat. I have seen this practice in many places.
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease