A team with a mix is best if it's not a mix just in terms of personnels but in projects and the way incentives are structured.
The culture within the team can go south sooner than one realizes if the 'process' types feel they're constantly cleaning up the mess of pioneers (who tend to get more attention in general for their shinny projects). And on the other hand the pioneers may perceive process-types folks resist any new changes and are stuck supporting outdated technology. The truth usually lies somewhere in the middle.
The culture within the team can go south sooner than one realizes if the 'process' types feel they're constantly cleaning up the mess of pioneers (who tend to get more attention in general for their shinny projects). And on the other hand the pioneers may perceive process-types folks resist any new changes and are stuck supporting outdated technology. The truth usually lies somewhere in the middle.