Why are political parties considered by some as a necessary evil?

Because political parties are the basement of democracy. Without them democracy wouldn't be based  on pluralims but on strong leaders and this may have bad consequences if these leaderships translate into dictatorship. Democracy is based on parties and many times these parties have to negotiate to get their goals and unfortunately, in these negotiations public interests are left out sometimes. As Winston Churchill said Democracy is the worst form of government except all those other forms that have been tried.