I don’t think you understand, it does make a difference as you’re being forced to take a FX rate on every trade. That is kind of like hoping lightning strikes twice (maybe not to that extreme). The bottom line is, when you want to buy or sell, you have to factor in both the FX rate and the stock price.
It is a lot easier to be able to deposit x amount into USD, y amount into GBP and just trade them, then convert them when you want to, rather than being forced to do it on every stock transaction (making you sometimes take a really bad FX rate in the short term).
The problem is, if you want to sell your stock and the FX rate is bad, it kinda puts you in a hard spot. Since if you don’t sell, your stock may go down and you will make a loss. If you do sell, you take a bad FX rate and eat into your profits (or even make a loss). If you had a multicurrency account you would just sell the share now and take the profit. Then when the FX rate is better (say a week later for example), you could convert the currency should you choose to. It makes a big enough difference, trust me.
EDIT: A good example is when the general election results were announced, the pound value went massively up. If your US stocks were 2% up, your profits were wiped at that point. If you predict your US stock is going to go down and want to pull out, you can but there goes your 2%. On the other side, if you sold your US shares on the same day and waited 2 weeks later for the GBP value to go back down, you could have easily done the currency conversion 2 weeks later and kept most of that 2% gain, rather than being forced to break even.