There are all sorts of reasons why people don't like the 'consecutive use of coupons' rule.
In my case, my trip was MAN-CDG-TLV out and the reverse trip back. I was unable to make the outward trip (in the end I flew Jet2) but wanted to make the return journey (so I could have bought a one-way outbound Jet2 ticket). However I was unable to do so and ended up throwing the whole AF ticket away and buying totally new trip with Jet2. Bit of a waste really.