Some mobile devices will run with a full node. They'll stake and everything.
Other devices (say, dumbphones) that can't run a node will end up using PoBC. The basic idea is:
- dumbphone node contacts other nodes
- provably establishes that the other nodes' blockchains are legit
- thereby proves that a given tx is legit
Why would you say that this is ill-advised? In my view mobile-friendliness is absolutely critical to mainstream adoption.
Why reinvent the wheel? Just use Electrum (open source!) or SPV.
Probably because XC offers both completely anonymous transactions and instant p2p encrypted messaging. In other words, XC offers a complete platform that protects one's rights to privacy. Bye NSA.