I agree with you that the value of the coins in your mobile wallet shouldn't exceed the value of the device itself. The problem is that you can have $500 in Bitcoin, for example, and in less than a year the price rises and you suddenly have over $1,500. I'm not a big fan of moving my coins if it's not strictly necessary, but in cases like the mentioned one shouldn't be lazy and should move at least a part elsewhere.
I dont' see any logic for this, honestly.
In general, I agree that storing bitcoins or altcoins on mobile devices like smart phones or tablets is risky but there is no logic that the fund stored in it should be smaller than the device value. I only concern that whether the device is new enough to almost won't have technical issues like suddenly dead and I can not access my hot wallets in it. Definitely, wallets always need to have backups so I will be able to access my wallets on computer in wallet recovery.
Very basically, mobile devices and hot wallets are for small funds, not big funds.