The thing is I don't think non-mining nodes add much security or decentralisation to the network. If you already are a major economic player, ie: an exchange, then yes, running a node adds security for both the network and yourself. Other than that I don't see much advantage. This is not a democracy, no one is gonna care if my node reject a transaction but if Bitstamp node rejects it, that has some real impact. On the other hand are nodes at the front of major pools which obviously have a great impact on the network but those do have an indirect incentive being a fundamental part of the miners behind.
if a full node was run just by mining pools. of course they would ignore their competitions efforts and just want their own blocks to win. so there needs a unbiased outsider group deciding what gets added to a chain.
normal home users that need to monitor just 1-5 addresses dont need to monitor every transaction of every block every day. and normal home internet would bottleneck the propogation if a home user was to foolishly want to connect to 120 nodes.
infact mathematically its better for home users to only connect to 10 nodes
however merchants that need to validate hundreds/thousands of transactions they are more needing to have a reliable ledger. so businesses are more important. because if they dont relay transactions of their customers then the merchant wont get paid if a pool doesnt add a transaction to a block.
where as users would prefer not to relay transactions and hope pools dont receive it. because they hope merchants would see a unconfirm and react, while hope a pool dont receive and confirm. so users are not actually needing to have a reliable ledger, users prefer slow networks because they hope the merchant act on the unconfirmed transaction so a user can double spend
with all that said..
if all users are just running core. unedited.. they are not decentralising th network. they are just distributing the chain
distributing and decentralising are 2 different things
