(Guess) All the UIA/smartcoin "data" is stored on the blockchain which will grow in size as the network develops. Transaction fees pay for the privilege of being included in a block and stored indefinitely.
I assume that the existence of a UIA/smartcoin will be verified at the point of a transaction. If the "lookup" on the blockchain of the desired smartcoin transfer fails, then the transaction is deemed invalid. (?)
A UIA that is deemed to have 'failed' will have had very little use, and therefore will take up some of the least space on the blockchain, so why bother pruning it? Any space it DOES occupy has been paid for in tx fees.
UIAs (smartcoins too?) require a fee to be registered. I assume this is to cover any hosting costs.
To avoid control-vectors I think that all entries into the blockchain should be irreversible. This means that extra-care should be taken to avoid problems further down the line.
For example the 'molecatcher squatter' (20k domain names squatted) and the (now solved) mistake of ridiculously high transaction fees on the stock exchange UIAs (Shanghai, Shenzhen etc).
These mistakes should be taken as lessons to further improve both BitShares the protocol and BitShares the community.
Startspirit, your posts are always very interesting - would you consider doing a Beyond Bitcoin hangout to discuss privatized smartcoins?
I feel that a personal discussion would better convey your ideas and I would love to hear a professional discussing existing financial instruments and how bts could take advantage of them.