I think the more cases to study the better. Based on my experience with foreign languages, the rules/grammar would not make a speech in a foreign language. The rules are secondary and the speech is primary. The rules would only attempt to find the consistencies in the speech pattern. As a result, a study of the rules along with the examples would mostly make sense to build the consistency patterns in the brain of the student. A student is learning a rule (i.e. a pattern) and then attempts to apply that pattern to create more examples of his own. Therefore, the formal language education is useful to make the student learn to create the speech the right way right from the beginning. The exceptions, on the other hand, are counter-productive in that sense. The students are asked to memorize them, but they do not help with the consistency. As a result, I think if there is a rule which would cover even the 1% of the cases, it would be much more beneficial to learn that pattern rather than memorize that as an exception. I think the way the modern Russian grammar is created is a mess. There are so many exceptions that it makes students wonder why do we actually need the rules? If I get it right, the 'exceptions' are just the rules (=the speech patterns) that the grammar book authors wanted to exclude from the study. With respect to what I said earlier, that kind of 'simplification' does not really make much sense to me. As a student, I would rather study the 15 cases and make my speech more consistent than recite those 'mnemonic poems' which help to memorize the exceptions. So, first, you have to identify the rule which applies, and then check in the 'mnemonic poems' maybe there is an exception! How, the heck that would help with creating consistent speech?! What do you think?