sometimes meet the human who learns or already knows a 5 languages , but if he speak to you on your language , you understand his speech is very terrible but he knows whole 5 languages, is it worth to learn the 5 languages and he later would be looks like an emigrant in all these countries ?
Are you sure the title should be multitasking and not multilingual? I'm sure if he desperately needed to speak like a native he'd use the foundation he has (even if it is terrible) and pick up on how to speak better later by listening to natives and so on. So, for example if he got a job in a different country and had to speak one of those five languages he'd start to pick up on grammar, slang, and dialect as he was there.
I think it depends on what your goals are and what kind of language ability you need.
If you want to speak like a native then concentrating on learning one or two languages probably is a good idea as no one has unlimited time to learn languages.
But it's not necessary to have that as your goal and sometimes it helps to be able to speak and understand many languages a little, even if you're nowhere near fluent and can't dream about being mistaken for a native speaker in any of them.