What makes you believe that is desirable?

the belief that wider representation in the government, less polarization, a strong parliament and a weak executive is more desirable.

A weak executive would seem to reduce representation in government though?

As each member of parliament must necessarily represent the biggest groups in their constituencies.

So a decent size group scattered across many dozens of constituencies into relatively tiny groups would simply never get taken seriously without a strong executive.

No, you've got that exactly backwards. Parliament is the representation. Because of proportional representation, parliament represents all groups in the country, and not merely the majorities in each district.

Exactly.

What? Parliaments obviously cannot represent every group within any given country.

For example, non-residents, tourists, diplomats, etc..

You're right. Parliament represents only the entire electorate. And only really those opinions that are shared by enough people to be worth an entire seat. Still, it's far better representation than you get in a district system, where only local majorities of the electorate in each district are represented.

So then in a country with a weak executive, who collectively represents the interests of non-residents, tourists, diplomats, etc.?

Because each group certainly has some real interests in common.