Yes, if the new ring was spliced in a figure-8 form to the exact mid-point of the old ring, then obviously the total load on the new ring would always be evenly distributed around the two halves of the old ring.
The problem arises if the new ring were joined onto the old ring close to one end. Go back to the example I used before: Existing ring of 120 ft. with a outlet placed 12 ft. from one end of it.
Now assume that a new ring feeding several outlets in the extension is tapped into the existing ring at that outlet in a figure-8 form (albeit a very lop-sided figure-8!).
OK, if the new ring is feeding just a TV, table lamp, etc. there's not going to be any problem. But if the new outlets are a kitchen or utility area, they could have a heavy combined load. Let's say that 6kW is connected to the new ring, giving a current of 25A.
Obviously that 25A will divide around the new ring, but as far as the old ring is concerned, it represents a 25A load at a point just 12 ft. from on end. Now suppose that the existing outlet at that 12-ft. point is feeding a 1200W load, bringing the circuit total up to the maximum of 7200W or 30A.
We now have a 30A load divided between a 12-ft. cable and a 108-ft. cable. The result is just 3A in the long leg and 27A in the short leg, which is greater than the cable rating. Obviously if the tap point was less than 10% from the end of the ring, the imbalance would be even greater.
I'll concede that in most situations an overload is unlikely, but it is possibl.
This perhaps highlights a general point about designing a ring which is seldom mentioned in text books: For best current dstribution, design the heaviest loads either into the mid-section of the ring -OR- put heavy loads likely to be on simultaneously at opposite ends of the ring to balance the current as much as possible.
Bearing in mind that in many kitcens the ring feeds a 3kW washer, 3kW dishwasher, etc. this can be quite significant. I would still rather run separate, dedicated branches for such loads.
