If a team is used to using Capistrano for application deployments, what is the compelling benefit of switching to MCollective, or adding it into the mix? The team is already taking on the task of adopting Puppet for system configuration management, so there is a question of how much to learn at once.
Capistrano and MCollective solve different problems. Sure, there is overlap, but Capistrano is very heavily focused on deploys, where MCollective is aimed towards generic orchestration - performing ad-hoc tasks and collecting information. You can make MCollective do your deploys, but IMHO it is better suited for discovery and ad-hoc information gathering, or for ad-hoc tasks that don't require guaranteed execution. I think both tools are useful to have in your toolbox. Follow your instincts and take it a step at a time; focus on adopting Puppet and then evaluate whether MCollective is a good fit for your use cases.
You might be interested in Puppi. This is a puppet module, a local command, and an mcollective agent wrapping it allowing you to do some deployment related activities.