As a History/Politics grad this was debated on a daily basis at uni. The political spectrum is a gross generalisation which is too simplistic to describe what are complex relationships. For example where would you place an anarchist on the political spectrum? It's impossible as some anarchists display so called Neo Classic liberal tendencies (extreme free markets) and some are more akin to classic Marxists. It is better to describe the characteristic features/beliefs of the particular "communists" you are looking at. The plotting of political beliefs is far too complex and time consuming for year 11s. This is the stuff of PhDs not GCSEs
It is rather pretentious to suggest that you cant introduce students to the idea of the political compass. (In fact you actually contradict yourself by initially stating it is too simplistic then later calling it too complex) You can use it in which ever way you want - teach at the level of the students. Why on earth would you want to introduce Year 11 students to the difference between anarch-capitalists, anarcho communists and libertarian ararchists. It is a good way of showing the classic left/right difference in a basic way which students can understand. You can use the political compass to show students where different ideologies fit on the spectrum - you dont have to teach it at 'PHD' level. I know because I've done it. Having a visual aid (such as the political compass) is an excellent starting point to then begin to look at the ideas that put a particular group on a particular point.