I've been using Simple for the past 6 months and I've got to say I don't miss my brick and mortar bank one second. These guys are the perfect example of doing it right.
From their site: "After 180 days (6 months) of inactivity, we charge a $5 monthly fee to avoid escheatment,..."
I think if you were to switch to Simple it would be very rare to not use your card for 180 days, but that's just my 2 cents. But yeah, I wouldn't switch to only have an "extra account" that would eventually hit that 180 day inactivity time.
The inactivity fee takes a long time to kick in--we're talking a year of not posting any transactions to your account. Their customer support is very open about it.