Well said economic reason are with fees I guess. When the size limit is reached on a block, transactions with higher fees are prioritized over those with lower or no fee. Or so, at least that is from what I understand with how bitcoin works.

I don't think it's a problem that some blocks might reach the limit during peak usage times. I would think it's a problem if a transaction could be delayed for more than a hour due to this though. Currently though, a lot of BTC... especially those traded for speculation on exchanges are all traded offchain and does not affect the blockchain at all. So this might be more of a problem the more people actually use BTC as a currency. Then again payment processors might cover this as well, but... inputs.io is probably still in recent memory amongst many of us.

But the bitcoin whitepaper does cover how to trim the blockchain down to a more manageable level (tl;dr, involves pruning old data after verification), and personally for everyday usage I tend to use a client like multibit to go around the space problem in general (though as a enthusiast I have bitcoin-qt installed as well).