SBD going below $1 means one pillar of support for the Steem price has been removed. Instead of it being profitable to sell SBD for Steem (maintaining the peg downwards) and SBD having more power to raise the price that way, it's now probably more profitable to convert Steem, which increases Steem supply and thus lowers the Steem price.
It is a minor thing in the big picture but it's definitely preferable for SBD to hover above $1 than below.