The electorate needs choice, and to know that the people they most trust to run the country are being put forward as candidates. People will be disinclined to support this system if they have no emotional connection with the nominees.
All parties have the right to be included in the system and to be confident that they will be treated as equally in opposition as they will when in power.
Finally, parties themselves must follow a democratic process of their own when selecting candidates – this must be the person chosen by following the right process, not favoritism or personal agenda driven.