If for whatever reason I have a total of £10k in savings and I want to know that a year from now, I can be sure of having £10k available if I need it, then I should save it as cash, should I not? And at the moment, there are better returns from putting it in a cash ISA than a regular savings account.
If I need to take it out in a year then so be it. It's not prevented me from using this year's ISA allowance for something else because I don't have anything else.
(This is all hypothetical, not my actual situation)