The plan to merge unions such as Amicus, the TGWU and possibly the GMB into one super-union sounds great in theory. But it's worth taking a look at past mergers.
I work in local government, and as a Nalgo member I voted to merge with Nupe and Cohse. The result was the creation of Unison, the country's largest union, in 1993. Instead of giving more clout to its membership, what we witnessed was the jockeying for full time positions within the new union. This led to the suspension of some former left wing Nalgo branches. Members were deprived of any contact with their shop steward because the regional offices had taken over the running of our branches.
Unions tend to want to merge for financial reasons rather than for better representation of their members. Old right wingers such as the late Woodrow Wyatt used to urge union mergers because he felt there were too many unions for the employers to deal with. As Kevin Devine says, a merger is one thing - ensuring that real power lies with the membership is another.