I'm with NeverBeat on this one.
Training is their WORK - have a laugh when the session is finished or in the gaps between sessions or whatever, but when the professionals are doing the training, the professionals take it seriously.
Its not like the 2 hours a week that us amateur footballers do where we have a laugh because its the only football we do.
Also, team spirit is based on much more than a bit of banter in training.