Should I consolidate my SQL instances?
SQL Express has many disadvantages. If you've already paid for a real license you should use it. You don't have to consolidate everything in one instance, but I prefer not using Express...

See http://msdn.microsoft.com/en-us/library/cc645993%28v=SQL.110%29.aspx

Max Compute Capacity
Max memory utilized
Max Database size
AND
No SQL Agent!