The whole point of POW is to secure the network with lots of hash calculations (hence the term 'work'). Phones have very little power (compared to asics, or even desktop cpu/gpu), so is it impossible for coins mined through phones to have any value and have a secure network at the same time.
I think it depends on how much you can mine. Theoretically speaking you can mine bitcoin with your mobile phone too, not like you are going to mine anything meaningful, it will probably have to be like a mobile phone pool where millions of phones all connected and maybe it can mine a a small number with help of some asics on top of that.
Hence, it is not about if it could be mined with mobile phone or not but its about if its making a dent in the hash or not. If couple of phones get together can provide a decent percent of the hashes even if at 1% of all hashes coming from mobile phone, than OP is right, its not feasible and not enough defense for someone trying to attack the network.
However, if it can be mined with mobile phone but its not practical and can't make a dollar out of it in 5 years type of low level than its fine.
