When bootstrapping Smart Treasury, ETH was chosen to avoid excessive IL, but stablecoin was close second. With ST pool on track to reach its 90/10 target on April 12th, now is a good time to throw a stablecoin into the mix and capture those IDLE/USD trading fees.
Also, Idle governance would benefit from some non-volatile assets at its disposal; budgeting and paying in IDLE or ETH can be inconvenient.
On the question which stablecoin to choose, there is a risk-based answer (DAI) and a fee-based answer (USDC).