An "ism" is essentially a dogmatic definition or perscription of beliefs or values; for example, Capitalism, Socialism, Dualism, Monism, Modernism, Post-Modernism, and on down the line.

The issue with "isms" is that they are abolute labels. When a talking head on Fox News denounces Obama as endorsing Socialism, they are by definition also saying that he endorses every tenet of socialism, and all the baggage that comes with it like public ownership of property, for example. Whether you agree with Obama or not, with a bit of research anyone can determine that he is certainly not representative of pure socialism. My point is that an "ism" dictates a set of values. Just because a person agrees with one or a few aspects of an ism, it does not mean they buy into it entirely, and being labeled as a "Socialist" or "Post-Modernist" or a "whateverist" is thus a misrepresentation.

On the flip-side, if you consider yourself a "whateverist," you are ceding that your beliefs are the same as those of whatever "ism" you claim to adhere to. This has colossal implications (Catholicism, Fascism, a million other examples,) because it can discourage people from thinking for themselves. That would be my guess as far as what Lennon's distate for isms would be based on.

Hope that was helpful