As a voter who will swing between Liberal and NDP depending on who is more likely to win the specific riding or election I'm participating in, I will never understand EITHER the NDP supporter nor the Liberal supporter who wants to do the other party in. Liberals who want to see the NDP fail and NDP who see the Liberals as more evil than the conservative option are self-defeating progressives. Splitting the left is what enables those who defeat progressive policy.
Ok here is the irony. Splitting the left defeats progressive policy. But uniting the left also defeats progressive policy. I explained this before.
When there is only one party left of the Conservatives it will be Liberal in policy even if it is NDP in name. The reason is that parties reflect political context and their supporters. If the Liberals vanished, their supporters would go NDP and make a party that is much more to the centre. If you want a progressive NDP you want the Liberals to take up the members who are less progressive.
If you want an NDP government in more than name then you need a more even split between the three parties and the NDP may win once in a while -- if they occupy a too restricted space they will not get elected.
Interestingly the NDP might influence policy when not getting elected than by having the Liberals disappear and the NDP move to the centre with the new members. In this scenario they become the centre party with no party to the left of them pushing them.
So best option is three-way tie with the NDP winning some times, or balance of power, second best is NDP at least strong enough to make the Liberals sometimes take their policies, worse is and NDP without a Liberal party and lastly a Liberal party without the NDP. The last two are not much different.
Another point is that parties should not be hypocrites -- that gets noticed. The NDP provincially should declare support to the Liberals for party status otherwise their position on PR looks like opportunism only. In a PR election the Liberals would have party status -- it is a freak of FPTP that they do not have it now. As I said before, I am fine with them conditioning this on Liberal support for PR but the offer should be made.