1. We can stop the job.
2. No-one can jump ship to another union to avoid striking.
3. Management can't use the fact of some unions not striking to slag off and isolate the one that is.
4. We get a sense of how powerful we are when we unite.
5. Members of different unions get to organise together, picket together, discuss strategies together.
6. The usual inter-union sniping tends to abate for a while!
It would be even better if we had ...
1. One union for all Tube (and transport) workers.