Repudiation is desirable (I believe SimpleX is actively working on it) ofc.

But the lack of it does take away the advantages of message timers in 99% of cases. It's a passive defense for when you are not being actively targeted.

Yes, if chatting with a fed and confessing a crime, it won't save you. 

 @simplex has always had repudiation on the e2e layer but from the handling of the SMP protocol you could construe non-repudiation and that's being fixed now.

Now what's a "message timer". 

 Messages that self-destruct after X amount of time.