Technically yes! but you can leave the harnesses in and use the back seats
For example when I drive to events fully loaded with tyres,tools etc the harnesses aren't used (but attached) just folded up so I can load all my stuff in the back, I use the OE belt on the road all the time anyway. Only once I'm unloaded and sprinting (or on a trackday) do I use the belts.
In fact I only use my bucket seats on a race weekend - all other times I put the OE seats in, so much easier than the faff and hastle race seats and harnesses give you day to day, why anyone chooses to use them for a road car is beyond me
The only benefit they give is feedback on the limit of slip which I don't do on the road.