its all theory but here is a story

mining pools will still be full node
in 2015 a tx looks like [txdata&sig]. in 2016 SW softfork would look like [txdata][sig] to a full node. so no worries about miners (i hope)

but it would only be [txdata] relayed/saved to a SWClient wallet..

what i could then do, is hack your SWClient so that im the only relay node you connect to.
i do this for 4 people just so your not curious about lack of network connects.
so i could make a [txdata] that is satoshi -> franky 50btc.
i send the [txdata] to you all. and because you all have the same [txdata].. you accept it (remember you cant contact a fullnode to check signature as you are in my hacked circle).
i then say 'bob im satoshi's friend as you can see he gave me 50btc,  i want to give you 50btc if you send me 5000LTC or $15,000.' your happy because its a cheap deal and also you think you will get fame for receiving funds originally from satoshi.. afterall the [txdata] shows that the satoshi funds came to me
you agree so i make a new [txdata] that shows franky -> bob.
you also see for other connections with the same [txdata] crediting you with 50btc
you then send me the litecoin/dollar funds..
i release you from my hacked circle, where you realise that the [txdata] is all fake.. and ive just run off with your litecoins or dollars.. all because you did not have the signatures stored to check locally while you were not able to check the real data.