When you convert a Lead, Salesforce creates an Account, Contact and related Opportunity (unless you check the box to not create an Opportunity). The Campaign Tracker automatically copies the tracking data from the fields on the Lead to the fields on the Contact.
Because the Contact will be automatically related to the Account and Opportunity upon conversion, by extension the tracking data on the Contact is also related to the Account and Opportunity. From a reporting standpoint, it is not necessary therefore to have additional tracking fields on the Account or Opportunity.
You will see that in the prebuilt dashboards (go to Dashboards and check the "Campaign Tracker" folder) there are already reports on Opportunities converted and won, which are based on the relationship to the Contact.
We put the tracking data on the Contact because it is really a record of what that individual did, and will be related to the Opportunity upon conversion.