Hilzoy's example is 'The Left'. Depending on context, 'the left' can refer to hard-core Communists or people more likely to vote Democrat than Republican. Either reference can be useful and understandable in context. The problem arises when someone uses it for one meaning while sounding like he is using it for another. This can be unintentional--if I speak of 'The Left' on the Social Security debate and you interpret it more as 'The Left' in the anti-communist battles, we aren't going to be talking about the same thing at all. It can also be intentional, as when someone tries to tar the 'more likely to vote Democrat than Republican' left with something said by the 'stridently anti-capitalist perhaps communist' left. This can also happen when somone mistakes the part for the whole--as in equating all Republicans with the most extreme members of the Christian Right. Even among the Christian Right there are differing extremes. I'm quite confident that most of you who dislike Pat Robertson would get along well with my parents who also dislike Pat Robertson. But my parents are firmly a part of the Christian Right, and the Republican Party, nonetheless.So am I committing the sin he speaks of by stating that a 'conservative' can't be honest, balanced, reflective, moderate, wise and decent? Yes perhaps. In the pre-W era I would definitely say that was true. And it is still true that there are lots of decent, wise and upstanding Republicans out there (McCain, Baker, Bush Sr. and (sometimes) Senators Hagel and Luger come to mind).
However, the Bush-inspired turn to the right seems to have silenced most of these decent Republicans, leaving near-crooks or idiots like Frist, DeLay, Santorum, Hastert and just about every other member of Congress as the GOP mouthpiece. As I see it, these people are re-defining the meaning of the word 'conservative'.
Incidentally, I suspect something similar happened to the word 'liberal' in the sixties and seventies. I wasn't around then (and if I was I didn't pay attention to US politics) but from the deeply ingrained attitudes of many modern-day conservatives (some of whom are respected friends) I can only infer that 'liberals' really pissed them off back in the day with tax increases, wasteful welfare programs and the like.
So maybe in 20 years we'll have a hard-left president who takes the country near bankruptcy, world wars and kleptocracy, and I'll be defending his ass vigorously just because Bush & Co. pissed me off so badly in the 2000-ties.