Um, I don't think you understand the function of a political party.
A political party represents people that vote for them, or people that think along similar lines (party platform). So if DA members in their official capacity, or as a group, were to be disadvantaged, then yes, they can start a court case. They also represent their voters in Parliament, and, as official opposition, can in theory affect laws being passed.
However, the court case in question is by the motorists of Gauteng. These may or may not be DA members, but they are not doing this as an official organ of the DA, nor are they doing this under the banner of the DA. A large proportion of the people affected by this do not vote DA, nor are they interested in the DA.
Therefore it's hard for the DA as a political party to claim standing, and to claim to be the disadvantaged party here. None of their member ordinations are directly affected by this, and large proportion of their members are not affected by this. Similarly, a large proportion of the people affected by this are not DA members.
OUTA represents the motorists of Gauteng directly, and as a result they have standing to bring the case.
However, phrase the question differently. Should the DA donate money and legal expertise to OUTA to fight this case, i.e. support them directly, and the case indirectly? Then my answer would be yes. They should have done this from the start. And I am unhappy that they never did. But at least the brief that they want to present is better than nothing.
