Pro Sites manual gateway

If we enable manual gateway, it appears there's just a contact form for people to submit payment details. We can always manage sites and give them access as needed from the admin section, right? Or am I missing the point of what a manual gateway is intended for?

The reason I'm looking at enabling it is because we have some clients with existing service agreements using an alternate gateway that isn't compatible with Pro Sites. I want to move them over to Pro Sites, but I don't want to require them to switch over to a new gateway immediately. I'm hoping to have them loaded up on Pro Sites, and when their contract comes up for renewal, they can renew using the new gateway at that time.

Should I use a manual gateway for clients that pay us outside of Pro Sites gateway, or is manual gateway even needed? Should I just load them up on a trial and extend access until it's time for them to renew? Any insight is much appreciated.