Hi Laxman,
MII gives you connectivity to all those data sources would run on OPC standards and other discrete relational data sources with brilliant dashboarding capacity.
The thing is, we have PI in most of the cases as a single point of contact that interfaces B-2-B systems. So if you have systems that can connect to scada and get data on the dashboards, you can choose whatever way you want to go just that the solution would be more scalable if you use MII.
It's completely a specific scenario that you can evaluate and suggest an apt solution.
Hope this helps you reaching there.
Thanks,
Swaroop