Not quite true, depending on the time scales we're talking about and whether you're a basic rate tax payer, and therefore have a £1,000 Personal Savings Allowance.
An example - if you can only save £5,000 a year and know you're only saving for the short to medium term, then it makes sense to put those savings into a 3% savings account, rather than a 1% ISA. There will be a nice graph to be made at which the money lost in tax (once your personal savings allowance is used) overtakes the benefit in additional interest, depending on the difference in interest rates between the two accounts, and the amount invested.
The closer you are to being able to fill your ISA allowance each year, the more likely it is you should use that allowance, regardless of whether you're using a cash or stocks ISA.
If you're investing for the longer term and therefore using a S&S ISA, then yes, you will need to keep it in an ISA wrapper, to account for future growth taking you over the capital gains tax threshold.