Different groups may benefit in the struggle for income, but this struggle may reduce total income. It is therefore preferable to find non-conflictual means to resolve disputes. Unions have a legitimate role and generally represent the interests of their memberships. Conflict is inherent in capitalist society. Efforts to eliminate it are to the detriment of the working class, as collective struggle is its only means for responding to the power the capitalist class derives from its control of the means of production.
Trade unions are vital to the defence of workers' interests within the framework of capitalism.
Can probably draw on many students' experience with some unions. They can balances between militant and backward members, portraying their position as the golden mean. I have found it important to stress that it is not the same position as that of economic rationalists. For an elaboration of these arguments see Tom Bramble's chapter in Rick Kuhn and Tom O'Lincoln Class and class conflict in Australia Longman, Melbourne 1996. A role play of a union meeting is the culmination of my course on political economy.