No, that is not true at all.

One scenario is the inability to book flights to certain destinations on the airline website. Another is if combining points/paid bookings. A third is if you want to book the long-haul flights before confirming all the internal flights. Another would be needing to add additional flights to a booking that has reached the 16 sector e-ticketing limit.

There has been times where I've booked trips of up to 35 sectors across up to 5 different tickets because the booking system refused to let me do it in 1.