That's a straightforward contradiction. The point of proportional representation is not to prevent single party dominance, but rather (and the clue is in the name) to provide representation in proportion to how people actually cast their votes. It's true that PR makes single-party dominance considerably less likely, because it eliminates the distortion of majoritarian systems which often produce landslide majorities for one party on the basis of 40% of the vote or less. But the bottom line is that if the electorate chooses to give one party 70% of the vote, a PR system will give that party roughly 70% of the seats. That's exactly what it's meant to do. The obvious example is South Africa - the PR system there has produced overwhelming landslide majorities for the ANC, far in excess of anything the SNP could ever hope to achieve, in election after election since universal suffrage was introduced in 1994. If you're a South African opposition politician, and you're disappointed that PR hasn't produced the parliament of minorities that it does in most countries, the rational response is to persuade people that your party has better policies and better leaders than the ANC - not to scream at voters that they're buffoons for failing to use the system in the way you think they're "supposed" to.
In our own system in Scotland, the proportionality is solely based on how people vote on the list ballot. The d'Hondt formula attempts to make the overall composition of parliament (constituency and list seats combined) roughly proportional to the popular vote on the list. It doesn't always fully succeed, because the seven list seats in each region are sometimes not enough to correct a really extreme imbalance in the constituency seats. But to the extent that the system is proportional at all, it's proportional to how we vote on the list ballot. Constituency votes can't contribute to proportionality - if they have any effect on the overall outcome, it's by detracting from proportionality, ie. by making the composition of parliament less reflective of the popular will. So it seems very, very odd for Liam to imply that people who vote SNP on the list are somehow not really voting for Nicola Sturgeon as First Minister. If you're electing a government or a First Minister on either ballot, you're doing it on the list ballot, not the constituency ballot. If by any chance the result of the election doesn't properly reflect how people vote on the list, that's a failure of the system (caused by the unwise ratio between constituency and list seats), not some kind of glittering success for PR.
But is that failure going to happen? In other words, will the SNP break the system by winning an outright majority before a single list vote is even taken into account? Liam seems to think it's a nailed-on certainty.
"Every poll shows that the SNP will clean up in the constituencies..."
Er, nope. What the polls actually show is that IF there is no change in public opinion over the next few weeks (during the most intense period of the campaign when opinions are most likely to change), and IF opinion poll methodology is bang-on accurate (highly questionable after what happened last year), and IF some very dubious "projections" of how constituency votes might translate into constituency seats just happen to be totally right, then the SNP may not need any list votes to win an overall majority. But there again, if it hadn't been for the Foinavon fence and the shockingly unpredictable behaviour of a couple of dozen horses, I'd have probably won a fortune on the Grand National last week.
Don't let me put you off, though. If you're an SNP supporter and for some reason you don't care at all about whether there is an SNP overall majority, by all means take a punt on your second-choice party on the list. Bear in mind that if the party you switch to doesn't receive at least 5% or 6% of the list vote in your region (an almost impossible threshold for RISE in particular), your so-called "tactical" vote will at best be totally wasted, and at worst will backfire by helping to increase the number of unionist MSPs. But hey, that's the nature of gambling.