No, you DO NOT have to have the disc in ALL the time. You can insert the disk, plan your route then remove it. But (& this is a big but) if you deviate from the route at all, you'll have to put the disc back in.
I know that's the theory, Bungy - and sometimes (but hardly ever!) it works. Maybe it's because our European maps are different, but several times I've been say 100 miles away from home, stuck in the CD and calculated the route. When it says you can start, change the CD for a music one, set off on the route, and not deviate, when it starts saying please insert the CD again; even though the road is exactly the one, unchanged, it's directed you down. What also doesn't help of course is the maps are years out of date, and the MoCo have made no updates available - even if it did reliably work as you say (and yes, occasionally I've seen it do that!) as soon as you come to a changed bit of road, it hasn't a clue.
You have to ask - how hard would it have been to stick an SD Card reader on the thing to load the maps from, without you having to keep swapping CDs...

Jim