I think you will find bmibaby didn't use F100's as start-up aircraft - they used 733's. They only introduced a few flights on the F100 as these were bmi regionals which flew EMAEDI etc at the time and did nothing during the day (they used to use them to fly to Malaga, Faro etc). baby using them would havcreated more money than leaving them on the ground, but the long term economics of it wouldn't have worked - the seat costs isn't as favourable as other types, which would result in them being less competitive thatn other airlines (eg easy, or Go as it was at the time).
They could work though for a new start-up from a completly new destination, but probably ony as the aircraft are very cheap to buy.......