As a way to learn about trading, it's a poor idea because the medium doesn't allow for serious interaction and debate. It's more about bragging rights for those at the top of the leader board. Chances are they'll have an agenda which won't be in the best interests of anyone following them. You'll learn a bunch of bad habits from people who like to impress - which won't serve you in good stead in the long run. As a sentiment indicator, if most of City Index' punters are long a particular market, you're best bet is likely to be to try and fade it!
As for copying other traders, that's fine, so long as you accept that the majority are wrong most of the time and the leaders displaying serious gains have achieved them through massive leverage and taking insane risks. You might make a few quid in the short term but, chances are, they'll blow up sooner or later, taking you and anyone else who mirrors their trades with them.