Pr is preferable if the following three claims are true

The three assumptions which prove that it is better to have pr than fptp:

i) Democracy is bad for the government
Democracy is bad for the government because it gives people a choice over how they are ruled. If there is a government then democracy is a constraint on its ambitions.

ii) Government is bad
Government is essentially bad because it has a legal monopoly on the use of force which other public entities (such as firms and charities) do not. There is never a requirement to be aggressive and ownership of people is false so governments are bad.

These first two claims establish that democracy is something good. If something is bad for a bad thing then it is good.

iii) Pr is more democratic than fptp
Pr is more democratic than fptp because with pr voters can vote for their favourite candidate without having to consider how the rest of the population will vote. When tactical voting comes into play then we begin to consider voting for people we do not like... which is undemocratic.

If pr is not good then at least one of the above must be false.