Let me get this straight, though: since everything is handled by smart contract, the only collateral is other tokens. So, as opposed to a traditional loan where a defaulted loan would result in repossession of a house or business, a default would transfer only the coins that were set aside.
I"ll assume that most people who take such a loan only have a small amount of collateral in coins (since otherwise they'd just use the coins instead of borrowing against their value - and if they can't be sold easily then they're not worth it as collateral either). Technically it's not an unsecured loan, but it will be only a fraction of the loan value. In that case, the interest would usually be much higher since the lender is taking a bigger risk.
Tokens are a collateral (or ENS domains). Actually it does no differ from traditional loan, since in the traditional loan in your example, the house or a business is the token! So in traditional sense the house/business ownership transfers to the lender, who then does the repossession. So to realize the house or business (the token) the lender must sell it in order to regain any losses. This is exactly the same pattern as in decentralized lending using ERC-20 tokens.
So what you are actually usually doing in real world with secured loans (loans with collateral), you do pledge value against value. That is the logic of secured lending, otherwise it would be unsecured. With the same logic shares, art, commodities are pledged, liquid or no-liquid, even if you have the market price and would quickly realize. I think the reasons why one pledged instead of selling comes down to different answers. With property, I want to own the token, even if I dont afford to yet. With shares, I believe that the company will grow and would not like to close my position since I might not get the shares back or back for the same price.
For me, a regular Joe, I pledge my tokens because I need more liquidity to participate in different ICOs. I get a decent salary, and I pay the loan amount back before it defaults. This way I can have more liquidity when needed (this was my personal reason why I wanted to use ERC-20 tokens for collateral). Also what I do quite frequently is that when I see the ETH price going down, I pledge my token portfolio to get a decent amount of ETH when it is down, and once I have my payday, I pay the loan back. For me it is just a quick way to get ETH when I need. But this is how I use my tokens and might not suitable for all. I dont sell my portfolio because my blood pressure would be too high if I would have to buy the same amount of tokens back from the market.