Mise
isle of lucy
The concessions made to factions within their own parties happen before an election, and are already present in the manifesto. Voters know exactly what they're going to get when they vote for the Lib Dems, say, because it's all there in the manifesto. Yes, it's true that many deals between Liberal and Social Democrat factions, for example, occurred during the making of that manifesto, and that the details of these deals are hidden. However, the result -- the manifesto -- is in plain sight; we know about it beforehand and we know exactly what we're voting for.Don't you think that happens already, between factions of existing parties? Just look at what happened to Labour in the 80s- they were torn between their own radical Socialist and Social Democrat minorities, and ended up making too many concessions to the former and so losing the latter, along with their realistic chance at government. Or the 2008 US Presidential election, when McCain was forced to throw the far right a bone in the form of Palin. It's not as pronounced, I'll grant you, but minority positions within parties can wield influence just like minor parties, and in a manner which is entirely invisible to the general public.
What happens in continental PR is that these factions shatter into independent parties, making their wheelings and dealings all the more public. I don't see why this is a bad thing.
With PR, you don't know what you're going to get. The deals all happen after the election, and there is absolutely no way of influencing them (unless you own a national news media, of course). You can't change your mind having seen the likely outcome; all you can do is watch helplessly as the party you voted for and believed in sells you down the river.
Of course, this is all happening right now, with our FPTP system. But at least at the end of it all, we have a local MP we can badger.