That is not the case - I'm in PCS, and on our one day national strike PCS rallies had many speakers from other unions, and as Serwotka in the monthly PCS Magazine 'View' stated the Trade Union Congress agreed at the request of PCS that unions should make defence of public services and solidarity with PCS a focus for the May Day activities.
I think PCS activists realise that their current campaign is going to be a tough but not totally insurmountable one, but one which would be obviously be more effective if other unions linked their concerns over pensions and below inflation pay offers, outsourcing etc with fellow unions. This won't be easy to achieve at all especially given other union leadership's usually ambivalent response to appeals of solidarity in action , rather than just solidarity expressed in words (as ever talk is cheap). However for all their faults which are usually discussed at great length by some u75 posters at least the PCS leadership are attempting to have a go.