It's not a dumb take. It very much happens.

However this happens in FPTP as well, they're just called wedge issues.

Obviously a view on abortion shouldn't dictate a parties view on tax. In FPTP it does dictate that implicitly where as in MMP it would be explicitly negotiated as part of a support agreement between two or more parties (to varying degrees, i.e. explicit suppport, or perhaps supporting a bill to the point it will be voted on while allowing a conscience vote by their members).