Participation rates in iSupplier Portal

Can anyone share their vendor participation rates in the iSupplier Portal?  Do you mandate that your suppliers sign up on the portal?

We have a approximately a 2:1 ratio of vendor to PO at any given time, so we are wondering if suppliers that are used only once a year will want to participate.  We assume so as it will behoove them to get in on the bidding process and do electronic invoicing and payments.  But we could use some factual information for our business proposal.

Thank you for your time.