For me, a general rule would be to never have more than $1,000 in a mobile wallet, and I don't think I've ever had more than $500 or so. The rest is in cold storage. I haven't ever had any problems, I haven't been hacked or anything, but just because it hasn't happened doesn't mean that mobile wallets are secure.
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.
It seems very logical, but from my own experience I will tell you that there is a kind of psychological barrier that makes you leave things as they are, call it procrastination.