That is the point, though initially the thought was it would prevent perpetual Labour governments. The system has only ever been ‘broken’ once when the SNP won a majority, every other time as the last session power has needed 2nd parties for support.
If they win a majority this time it would be the second time they have broken the system. More likely they will need Green support again though.
Although if they are only 1 or 2 short there may be other options for partners on a policy by policy basis.
and @mancity2012_eamo
There are basically only two ways to win a majority. Win it all through just the constituency seats and no regionals. To do that a party would need to win over 91% of those seats, and then the fact they can't win any at regional doesnt matter.
OR, win the perfect storm of circa 70% constituency seats and circa 30% regional, which is about the only possible combination before any more of one cancels out the other. And obviously depends on other parties in the regional being relatively evenly split. Which is what happened in 2011.
Currently it looks like SNP will win between 85-89% of the constituency seats, which is enough to stop them winning enough at regionals to bridge the gap, but not enough for an outright majority. They could well end up with 60-62 constituency seats, and only needing 3 or 4 at regional level to get over the line, and because they did So well at constituency, they will have no chance of those 3 seats at regional despite having close to 50% of the vote.
A good example is the south region, where in the 2016 election they got 3 of their only 4 regional seats. Now, because two of the constituency seat gains (ayr and lothian) are both in the south, they are likely to lose 2 or even all 3 of those regional seats, with the same or higher percentage of votes, because the dividing factor is now 2 higher.
All that being said, I personally don't think the system (as perverse as it is) is THAT unfair. In terms of preventing a majority, sure, of course it is. But in terms of fairness and representation, by the time the two systems are combined, they kind of balance each other out. If you look at the overall resultant make-up of parliament relative to the overall voting percentages, it is broadly proportional by the time it all works itself out. Far more than Westminster.
I guess the only true PR would be to do away with constituencies and regions altogether and just go for a straight percentage of vote to percentage of parliament. But changes a whole lot of other things and might not be operational really.