its a problem for widespread acceptance, until either the blockchain isn't required for "reference" use (ie make Multibit the reference)
You do realize that multibit cannot run at all without thousands of people running full nodes, right? And that if the network consisted entirely of multibit like clients it would have no security at all...
It's fine that users with limited resources use thin clients, but— they're just thin clients— someone has to run the servers for those thin clients, and if its only a few people then they can tell you whatever they want and the house of cards comes tumbling down.
so, like the network of miners underpinning the whole system. they can become the trusted full nodes (in their interests to protect the integrity of the network). if Multibit (or similar) is not a solution, then what is? over months i see this issue of blockchain size raised and kicked down the road by people saying its not a problem: hard disk space increases, bandwidths increase, someone will do a pruned client. too many techies not understanding ordinary users restrictions and limitation. this could be the largest obstacle to adoption, expecting people to take multi-day, multi-GB downloads is not going to work.
Yes, your point is valid, but the strength of a decentralized peer-to-peer network relies on its peers, I have no problem running a full node 24/7, many people may not be able to do this for various reasons, using bitcoin on a smartphone would be one.
A user choosing not to run a full node is trusting the security of the network to the users who do run it, it is not the miners who control the network, all the people who run the full node control the network, every node is a vote.
Eventually there might be a way to shorten the blockchain, but as it is today it is not a problem, many people have more than 10GB of porn on their smartphones, on computers hundreds of GBs in porn, I think we can afford a few dozen GB to the blockchain.