How much do excessive conference meetings help employees deliver the best to the organization?

Like any meetings, the purpose, agenda and rationale for the meeting needs to be well defined upfront. Excessive conference meetings obviously waste time for everyone. Communication is important but having too many meetings can lead to time not being productive. All the outcomes and actions from each meeting needs to be captured and followed up on.    

Define "excessive" ?   "Too many conferences" ?  or Conferences that have excessive content and so not targeted to build individuals?  The fact you mention excessive suggests they're not adding the right value, and therefore won't enable, engage or energise your employees.

Not very much, in my humble opinion. Time in large conferences can be spent team building across functions...or actually executing on the work.