Keeping them all in multiple wallets will ensure the risk of losing all your money goes down but the risk of losing some money due to forgotten/lost seeds goes up. So, really, you are trading one risk for another.
Someone irresponsible enough to lose their seed phrase or forget their password can do it with a single wallet as well.
So, the risk of losing their assets will be higher for such a person if they have a single wallet. Imagine you have $1,000 in Bitcoin kept in a single wallet, and you lose the seed phrase or forget the password of your wallet; all your funds are gone for good unless you have some backup, etc. However, if you have that money split if maybe $333 each in three different wallets, you lose one of them but have two secured, you won't lose everything. 
That being said, it's generally irresponsible for someone to be in that situation. If you can't keep your wallet(s) secure, cryptocurrency is probably very risky for you because you will always be afraid of losing all your funds for being reckless, but if someone is stubborn and still wants to get involved, it's better for them to have their funds split in different wallets and seed phrases for each should be kept separately with backups as well if possible.
Newbies often lose their funds due to their reckless behaviour with their wallets and seed phrases, apart from getting phished or scammed.