Yep. Until
DCL allows placeholders to be converted to cruises online and/or fixes the excessive wait times, it will be very helpful to use a
travel agent, so they can wait on hold instead of you.
If you are on a cruise booked by a travel agent and purchase a placeholder while onboard, you can assign the placeholder to that travel agent. If you didn't use a travel agent, or if you want to change travel agents, you can book the placeholder directly through DCL and then move it to the agency of your choice within 30 days of the date you booked the placeholder. I haven't done this, but I think you just need to give the agency your placeholder number and they can call to get it assigned to them.
When you are ready to book a specific cruise, you can ask your travel agent to take care of it. I just send my travel agent an email with the cruise and stateroom category I want and mention that I want to use my placeholder, Then she calls in, waits as needed, and emails me the booking confirmation by the end of the day. I don't know if all agencies work this way, but large Disney-experienced agencies (including
Dreams Unlimited) should.