Has to be to solve time-wasting for me. Seen some teams doing it from kick off but refs only ever take an interest in time wasting from the 85th minute onwards. So, yeah, an independent time-keeper please - I bet Fergie would retire the very next day if they did bring that in. But we all know that the ball is in play maybe 30 out of 45 mins in a standard game, so would have to be strict guidelines on when to stop and start the clock, otherwise we;d all be going home from a night match at midnight.
Other than that, as mentioned above the injured player leaving the field is farcical. So how about a new rule to state that if a ref stops a game for a player, he can't return for 3 minutes after leaving the pitch. As well as giving the physio the extra time to effectively patch up genuinely injured players, it may also deter those cheaters who do it just to waste time or influence the ref's next decisions (he's hit me once ref and forced me off, you gonna let him keep doing that? If I get injured I'll have to tell the insurers that I did warn you.). Again, the 3mins would be recorded and signalled by an independent time-keeper, but can only apply to one player at a time - otherwise you'd go out to crack a few opposition players all at once knowing they'd all have to sit out the next few mins.