You do not need a separate FormsCentral account.
You can logging to FormsCentral for Acrobat (desktop app installed with Acrobat XI) with your current FormsCentral account. You will need to keep your FormsCental account if you want to be able to keep distributing forms and collecting data. FormsCentral for Acrobat without a FormsCentral subscription can only generate PDF and cannot collect data.
Hope this helps.