Knowing the going rate of something is essential to deriving your own valuation for it, and for liquid assets the personal valuation is the going rate - plus or minus epsilon to account for friction and personal needs and beliefs.

If I meet you and offer you 1000 bitcoins in exchange for $X, how low will X need to be for you to accept (giving you some time to complete the payment)? I doubt the answer is much different from $12150 (to within ~10%), where $12.15 is the Mtgox last traded price. Because you know that even if you have no need for bitcoins, you could sell them for ~$12150 on the open market (and that if you needed bitcoins and I didn't come to offer them, you could buy them for ~$12150).

So public knowledge of exchange rates is important. And actually traded rates aren't reliable because someone can trade with himself to manipulate price. Only a committing public offer can serve as an indication of the market rate and depth.

Bitcoins and dollars are not stocks/shares. So a stock market is different from a currency exchange.

If I want to know the value of a company that owns 1000 dollars, 1000 bitcoins, 1000 devcoins, 1000 i0coins, and 1000 ixcoins then I know right off the bat that is is worth that basket of purses of currency. If it also owns some land, a few buildings, some furniture, and a million widgets, then I still do not need to know how much shares in the company last sold for nor how much people are offering for the company in order to evaluate how much I think that land, thouse buildings and those widgets add to its value.

So basically a company is a basket, and I can evaluate the contents of it and apply my own situation, since some people looking to buy the whole company might be mostly after the widgets and not be much good at finding good prices for land and buiildings or not have time to dispose of them at a goood price thus be forced to sell them quick at firesale prices, and other people looking to buy the whole company might be very interested in that land and those buildings for some long term plan but have no interest in widgets.

So the whole thing is likely to be of wildly different value to different people, whereas each of the components inside it might have some ideal disposal method or market whereby the best possible price could be gotten for it. So someone expert at breaking up companies into saleable pieces might have yet another value the company is worth to them.

I think if we have excellent details of precisely what assets the so called company actually consists of and most of those things have markets, whether scrapyards or pawnshops or secondhand stores or whatever, we can likely get a better view of how much it is really worth than if we believe the hype of all the fans who think it is special in and of itself quite regardless of whether it owns two dimes to rub together, simply because it is so cool or it was founded upon a clever idea, or because if its manager doesn't get headhunted it, rather than whatever company headhunts that manager, should be able to turn a profit this coming year...

Obviously for bitcoins we do not know how much "reserves" "it" has with which to back itself - how many assets it owns - so we cannot take that approach. But with companies we specifically do want to know how much we could get for its assets if we liquidated it don't we?

-MarkM-

The difference is whether they represent value, or represent a real thing which possibly might be amenable to being assigned a value.

We know a dollar is worth a dollar, we know a satoshi is worth a satoshi, we know a bitcoin is worth a bitcoin.

What we don't necessarily know is what a company is worth, at least not until we know how many satoshis it owns how many dollars it owns how many bitcoins it owns, and how many satoshis, dollars and bitcoins we can get from a salvage company for its chattels buildings inventory etc.

-MarkM-

I still see the "shares aren't currencies" argument as irrelevant for the technical implementation.

In any case, to avoid the "non enforceable orders" you need binding advertisements (with an expiry block) and for that you definitely need to modify the protocol. If we did, I would also prefer to implement ripplecoin instead of just using colored coins: more divisibility for non hostcoin assets, not needing to move satoshis around.
At that point you probably also want more divisibility for both the hostcoin and the other assets.
And of course the hostcoin, being cash (a scarce money) should have demurrage.