Unions are essential to maintaining a balance of power. It is a mistake to think that legislation is sufficient. When faced with the prospect of walking off the job and suing your wealthy employer for breach of contract or knuckling under and just accepting that they've screwed you out of salary, pension or whatever, the individual will almost always keep the job because they have a mortgage to pay and children to bring up, they can't afford the legal fees and they can't deal with the stress of a hostile action which involves them personally but others perhaps only in a professional capacity. A union can provide the legal backing you need, which often needn't be used because the mere fact that you have it tends to make the employer behave more reasonably. I say this based on recent, personal experience. My employer was a large plc and we were transferred to a small firm that had bought out a part of the business. Legally, nothing should have changed but the name over the door. In practice we had to sign compromise agreements in order to keep our jobs, and these agreements accepted the removal of any part of our terms and conditions that the owners of the new business didn't want. (The cost of our terms and conditions came out of their pockets in the new regime, so they had more of an incentive to cut costs than they had previously.) Did the HR department of our plc employer protect us? On the contrary; they failed to answer our questions about it during the 'consultation process' and wrote up minutes of meetings in a way that didn't reflect half of what was said. The junior member of staff who was provided to us was very embarassed by the whole process. No-one in a senior role in HR deigned to talk to any of us. Basically the MD of the plc had done a deal with the boss of the buy-out company and nothing was to get in the way of it. 
Membership of a union would have given us both a negotiating position and a trained negotiator to help us. As it was, our only alternative was to effectively resign in the midst of a recession and at a time when most of our industry had moved out of our city so that there were virtually no jobs for us there. I say 'our', but of course we were each an individual, so there was no 'us'.
Without unions very good employees might do well, but that's no reason to condemn the vast majority of hard-working people to powerlessness in the face of ruthless employers, most of whom are interested mainly in making money, which means mainly keeping down costs, sometimes with complete disregard for the law.
Public sector workers are in a similar situation when their services are outsourced and costs are cut to the bare minimum, which means cutting terms and conditions as well as cutting standards of service to the public. (Somebody's making a good living out of the privatisation of the cleaning of hospitals, but it's not the cleaners, and nor are our hospitals nearly as clean as they used to be.) The public service organisation is happy just to get the cost of the service off their books, and if they are interested presumably just cross their fingers and hope that no-one will complain too much about the deterioration in the new, more efficient, private service.
I'm all for reform of the public sector, but I fear that it might be done in a way that makes money for a few people and does little to improve the efficiency of public service.
I think that the problem is the narrow experience and education of most of the unions' leaders. Like anyone else (bankers and their bonuses? hospital consultants and their bonuses?)public service workers are reluctant to give up what they've already got, which makes reform of the public service difficult but not impossible. I'm not arguing that public services should not be reformed, nor that they couldn't be made to work much more efficiently. I also think that people who work in the public service rarely have any idea of what it's like in the private sector,especially in a business in which every penny is counted, but then you often find that sort of disparity even between departments in some companies. This lack of understanding of how the private sector, at its best, operates is what prevents unions from providing the best sort of leadership to its members. Maybe unions should require their leadership to have done courses in business management, economics or accountancy, not just to have read up on labour history, enlightening and worthwhile though that is.