First, it is not an alternative system to our current first-past-the-post electoral system. It is a generic term, encompassing a large number of systems, each of which has its defects. It is not a case of FPTP v. PR. Any debate on replacing the existing system has to be in terms of FPTP v. STV, or FPTP v. AMS, or FPTP v AV+ and so on.
Secondly, the term is a narrow one. The proportionality relates only to the relationship of votes to seats: if you get 10% of the votes you get (more or less) 10% of the seats. It excludes the impact on negotiating power in the House of Commons. 10% of votes = 10% of seats does not then equal 10% of the negotiating power in the House of Commons. It can result in a party with 10% of the votes/seats exercising disproportionate negotiating power. That is what we are witnessing at the moment. Roughly one in five electors case their vote for the Liberal Democrats, yet the Liberal Democrats are now pivotal to determining not only who will be in Government but what policies will be pursued.
Under our FPTP system, the present situation is exceptional. If the electoral system was replaced with one of the PR systems, it would likely be the norm, with post-election bargaining, the generation of policies not placed before the electors, and with no collective accountability of government at the next general election. It is not an edifying prospect.