At the risk of sounding pessimistic, and as someone who also identifies himself as leftist: If the end result is voting between black/white binary choices, and that act of voting is itself one of the most important self-expression, does the fact that I disagree with them in a few points matter?
>does the fact that I disagree with them in a few points matter?
Perhaps not, but you're also lending legitimacy to a system that is abusing you.
But this isn't a board game that you can quit and go home. You are subject to your government rules regardless of if you participate or not. So it is probably better to try and get political representation that you agree with 60% of the time, rather than one you agree with 5% of the time.
> and that act of voting is itself one of the most important self-expression
That's what lobbyists want you to believe, at least. It makes their job a lot easier if they are the only ones carrying out democracy.
You need to select someone trustworthy enough to not botch your message, sure, but usually all political parties put up people who are trustworthy enough. What is much more important is your expression to the hired after they are on the job. That is the only way they are going to know what you are thinking. They are not mind readers, surprisingly.