Italy's strange party block system is a weird one, that I must admit. Multiple parties make it more likely for one to find a party that's closer to his political stance though.
Totally, which is a strong vote in their favor. Although in principle it's not too difficult to replicate something similar with only 2 parties, it just makes the primary much more important. That's how it is here in NYC -- it's a single party city, so the Dem Primary is where the real action is. And, there is wide variance in the local Democrats on many issues.
In practice, though, party discipline creates pressures for more homogeneity. If you don't "go along" with the other Democrats (Republicans) then they won't go along with you and you can't get anything done. But, the same sort of thing happens in multi-party systems, it's just that you end up creating a cross-party coalition rather than an intra-party one. But, the pressures are bound to be different, e.g., the US committee system.
Also, note that most of the multi-party systems are parliamentary ones, which brings a whole other set of things. The parties are much, much more important there b/c they form the basis of voting blocs that have relatively untrammeled political power. Divided government, in the American sense, doesn't happen.
But you do vote for an individual in any given constituency and each constituency gives a single MP. How is that tied to a party?
If I'm recalling correctly -- and I'm not an expert about UK politics, so most of this is by osmosis -- the parties in the UK are much freer about shuffling their members around. So, for example, Labour will put one of their celebrity MPs in a weak Labour district during one set of elections b/c that will help them win the district, while putting a weaker MP (e.g., one who has recently had a scandal) in a much safer district.
It's hard to imagine that sort of thing happening in the US. Kirsten Gillibrand has developed a New York constituency and a reputation on a series of issues important to New Yorkers. Not only would she most likely have to move if she wanted to run for a Senate seat from Arkansas (residency requirements for such positions are determined state by state), but her legislative and political record would often be at odds with the local constituency.
A lot of those differences may be overdetermined, though. The UK is a much smaller and more homogeneous country than the US, and it lacks our (admittedly somewhat bizarre) federal system. So, those kind of distinctions might be products of those differences rather than MP shuffling. Or, they even could be the cause behind MP shuffling. Likewise, despite being FPP, UK has a parliamentary system.
My only real point of the above rambling, to the extent I even have one, is that proportional representation (which is what you're really suggesting) isn't a silver bullet. It has its own pathologies, and we have to weigh its against FPP's. Although I think it probably has more merit than FPP as a system, I'd expect that even if there were PR you'd probably only see a handful of parties that are real contenders. So, query whether you'd really have candidates that mirrored your ideology/political views.
And, actually, my real point was just the one Kajhera made better than I did: you want to have the most impact with your vote you can have. Voting "sincerely," the technical term for what Tshern is advocating, rather than "strategically" is often not in your (or, debatably, the nation's) best interests. And, as I indicated above, even in a PR system you're almost assured to have to do it, too.
Unless you, personally want to run for office that is ...
