For some, no doubt. For others the extra ten minutes on a half-hour journey may tip them back into their cars. There are always winners and losers, but the bulk flow is to the city centre.
Whilst I agree with you that the bulk flow is to the City Centre and this will remain the case. I would disagree that the extra ten minutes, if that is the amount the journey increases by, would have that drastic an affect as the half hour journey on the North Berwick service gets you to Drem easily enough.
Many people ditch their cars not just because the train is quicker but because it works out cheaper than the cost of fuel and the cost of parking in Edinburgh City Centre combined. It also has the benefit that this half hour is your own time e.g use you phone/laptop or read a book etc.
With the advent of the 10,000 or so houses that Musselburgh, Prestonpans and Wallyford is about to get swamped with, journey times on the A1 and through Musselburgh are about to get worse because there is no planned Road infrastructure improvements to go along side all this building.
As I had said earlier I doubt all services passing them would be calling at these stations anyway because the way the East Lothian services are going I can see the day of peak services being almost every fifteen minutes to keep up with the demand to the City Centre.
Keep in mind that stations at Portobello etc would have the draw of residents commuting to other sides of Edinburgh such as Edinburgh Park, South Gyle and Gogarburn or further afield, where currently the thought of them getting into Edinburgh to then get a train to their destination is less appealing than driving either cross town or round the bypass, both of which are time consuming and fuel heavy.