I think the reason you have that credit is not to speculate that in 5 days the stock will go up. It is to allow the buyer to buy more shares without freezing cash with the broker until a transaction happens. If a person cannot understand this then it's a good thing that dumb people lose the money that they shouldn't deserve.
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In USA, Australia etc. they have the T+3 settlement. I heard USA used to have T+5 but changed it to T+3 some years back.
T+5 is definitely not made to so that trader can buy and sell within 5 days and a make profit although some people can try it. And it's also evident T+5 is not enforced by the brokers these days.
The other alternative is to have the cash in the CDS account before buying anything which means you have to keep the cash with the broker until you get a deal. That's a kind of waste of your funds. So using your existing portfolio as a collateral with this T+5 rule you don't have keep the cash with the broker.
Or are there any other reason for having a T+3 rule?