Most of the arguments against voting point out (rightly) that it achieves nothing. This is reflected in slogans such as "if voting changed anyting, they'd abolish it" and "whoever you vote for, government wins." I'm writing this post to focus on the other problem with parliamentary democracy - namely, that it is worse than useless.
However, despite often agreeing that the ballot box doesn't change anything (even if their literature and party leaders are happy to say otherwise), leftists argue that elections serve as a way to galvanise people. It is one of the few times when many people get political, and gives us an opportunity to engage with them and draw them into campaigns. Not to mention that socialists in parliament can use that position to campaign and propagandise around what's occuring outside of parliament.
Except that getting people to meetings, knocking on doors and so forth can be done just as easily around an issue as around a candidate. In fact, it may even be better, because it gets people talking about ways to tackle a given problem away from the ballot box and offers an opportunity to engage on the idea of taking direct action.
Too many people have illusions in electoralism, and to be frank it's one of the reasons that a mass direct action movement hasn't taken off in the UK. The entire left pumps its money and resources into chasing votes because their focus is party-building, and that way lies new foot soldiers, more subs, more funding to pay full timers, and more paper sales. It also reinforces the illusions people have in voting, because whilst leftists in theory have a (slightly) more radical end than liberals, they advocate largely the same useless tactics.
Meanwhile, those movements and organisations based on direct action remain tiny and marginalised. Both because of horizontal organisation and because of direct action as a tactic, we are able to punch well above our weight - workfare being one contemporary example - whilst the left and unions punch well below theirs. But imagine what we could do if, organised in the same manner around the same ideas, we numbered in the thousands?
Me and you might not have the resources to do that, nor most anarchist groups, but that's another problem with illusions in elections - those with the resources are pouring them into voting campaigns. That's why electoralism is so useless. Not only does it not change anything, it draws massive amounts of time and money away from the things that do.