According to the Steem Whitepaper

If a token is viewed as ownership in the whole supply of tokens, then a token-convertible-dollar can be
viewed as debt. If the debt to ownership ratio gets too high the entire currency can become unstable. Debt
conversions can dramatically increase the token supply, which in turn is sold on the market suppressing
the price. Subsequent conversions require the issuance of even more tokens. Left unchecked the system
can collapse leaving worthless ownership backing a mountain of debt. The higher the debt to ownership
ratio becomes the less willing new investors are to bring capital to the table.

The high SBD price is good for the long term, but as the whitepaper says, will make Steem unstable because a higher SBD means you can generate way more Steem tokens, thereby significantly affecting the price and supply of Steem.

Also,

If the amount of SBD debt ever exceeds 10% of the total STEEM market cap, the blockchain will automatically reduce the amount of STEEM generated through conversions to a maximum of 10% of the market cap. This ensures that the
blockchain will never have higher than a 10% debt-to-ownership ratio.

Having a high SBD relative to Steem would increase the amount of debt on the network. If the price is high, people will be trying to get as much SBD as possible and then cashing out.

Having it worth $1 USD adds stability. Sure, you could have it worth more, as long as the total value does not exceed 10% of the Steem marketcap, but then you can't really control it as well. The blockchain will not allow you to convert the SBD to an equivalent value of Steem. As a result, I guess people will just send it off into other cryptos.