They're fucking stupid.
This is the issue tho. You seem to think that one needs to make a vote "valid" by voting a successful party. If everyone followed this train of thought, the USA (for example) would be doomed to eternally just have democrat vs republican parties government. Here in Spain we've essentially had the same up until recently: red (psoe) vs blue(pp). The brave people voting against the norm for what they believed was correct made the arithmetic much harder, but at the same time showed there's strength to be had in third parties.
Example:
Imagine Bernie as a third party, and now imagine he's needed for the democrats to govern. He could push his socialist agenda deals in a way he only could as the sole winner for the elections. This is pretty much it, smaller parties being able to make political deals so issues that might be less mainstream get the spotlight too.
PS: As long as you only have two real positions, you're gonna keep making two camps that stray further away from each-other due to extremists, imo.
And if you don't think Spain is as divided as the US, just a reminder that we had a big civil war on 1936-39 and a dictatorship that lasted almost 40 years.