yes you can surely start trading from 0.5 Bitcoin and i do not think that 0.5 bitcoin is less amount for to start trading and i am sure you can get a maximum profit through trading by 0.5 Bitcoin .
The majority of the people here don't have 0.5BTC for whatever purpose. If you talk about people using low amounts for their trading sessions, then it's even less than 0.05BTC in some cases. That's why they mostly jump into altcoins as they are subject to regular pumps and dumps where some coins gain like 100% in a matter of days. I personally find that it is more gambling than actual trading, but it's their choice to do whatever they want with their coins. High risks means higher possible profits, but it also means that the losses can turn out to be way more severe.
No matter if they trade
BTC/fiat or
BTC/alt
In my opinion, this is the best about trading on Bitcoin exchanges, that you have the possibility to trade with very low amounts of money on good fees (0.15-0.25%). In real markets you'll never get that chance. You can even start leverage trading with 100 US$. Try this on any CFD market with lots of 100
BTC or 10000 US$ per lot and minimum size of 0.01 lots. Impossible for people with very low amount of trade money to start trading there.
//alex