or starters, it forces the party to spend an inordinate amount of time and resources on what is really an intramural battle, the wounds from which may not mend in time for the general election against the real opposition.
Worse, voters have proven themselves ill-equipped for the challenge of sorting the wheat from the chaff during primaries. The main civic function of a party label is that it provides voters with information about what a candidate might do in office. Most voters, even if they do not pay much attention to politics, have a basic grasp of what the Democrats and the Republicans stand for. So, in a general election, they have the requisite information to make the choice that reflects their interests. But party labels are meaningless in primaries, so low-information voters sometimes respond to the candidate who spends the most money, says the most outrageous things, or has the highest profile, whether or not he is a good choice.
Moreover, turnout in primaries is abysmally low, which can give an advantage to ideological purists and extremists. Primary electorates amount to just a fraction of the voting-eligible population, so high-interest ideologues, though relatively small in number, can nominate a candidate who is either too extreme to win the general election, or who is ill-suited for the natural give-and-take that should attend actual governance.