After some exploration on this and a couple of other websites I found some information on the TT card that makes it seem worth the expense.
Our original quandary was making a timely escape from Heathrow to Bath via train using an advance fare but faced with the possibility of loss of fare if we miss the train. The next type of fare is the off-peak which is more costly but the cost looks like a wash if we go the TT card way.
Plus we are planning a day trip or two out of Bath, then will need a return to Paddington. The discounted fare savings add up to cover the TT card fee and exceed if we purchase the off-peak tickets out of Heathrow.
Other than traveling after 0930 are there any other significant downsides? I didn't see anything objectionable on NatRail.
Am I right with my analysis? What did I miss?
Any report on how the TT app for Android phones is working? In early posts it wasn't working well.