Yes, you are right, Bitfinex has given us a lesson that we should avoid to keep our coins on the exchange. I am surprised that it is second time happen so why people are doing it. Personally i refer to safe my bitcoins into local wallet and i believe it is more than better Bitfinex.   

But for the traders is want to think again, maybe not their intentions for believing with exchanges but there considering about every fee they will get in every transaction in trading they're made it.

This is not always practical even web wallets were hacked several times or they were ran away scamming lots of people. And also traders have to keep some coins in exchanges to do daily trading and profit from it so even though they know it is risky they will keep almost all they have in exchangers.

That could be a problem for arbitragers and traders who don't want to miss up on opportunities that could pop up anytime in the market. While the risk stands when keeping your coins in an exchange, also, the opportunity might slip if ever there would be some significant movement in the market and you don't have any coins on the site. As a personal suggestion, I think it would be better to just keep all profits in the site AND withdraw your initial investment if you deem that your profit is already enough to continue your trading operation. That is what I'd do if I was a trader.

Web wallets are also not safe for holding a lot of bitcoins, if you know you have a large number of bitcoin then you should not take the risk of keeping them in a web wallet but keep them in a hardware wallet like electrum or bitcoinCore etc, or just split them into parts in different wallets to be safer because if you get a problem in one wallet you wont lose all your coins at once.

People love to keep coins in an exchange because you can keep many coins in a single amount. Making an account of every coin is a tiring job though but people should prioritize the safety of their coins first

No one needs to have accounts of every coin they trade, because people usually buy different coins for trading them and they just sell them for bitcoins when their prices are up, so someone would just need a bitcoin wallet for keeping his funds.

I think, should be enough just to exchange BTC on-demand use. And I have kept since the loss is unlikely to recover.

That's true but you can invest there for trading but make sure that you withdraw your profits on regular basis to minimize the risks.