That's true; even with metro frequency services in western Hertfordshire, I normally plan journeys into London around specific departures if I have an advance ticket (especially if not from Euston, St. Pancras or King's Cross), and that's even more so the case with a twenty minute walk to a station.On the other hand, if you have compulsory reservations then you have to make sure you turn up early to avoid being delayed en route and missing your train – especially if you are arriving by road and so subject to the vagaries of traffic and/or unreliable buses. Whereas if you know you can just get on the next train, you might not be so bothered about cutting it fine and arriving with a few minutes to spare.
Has there been any indication that season tickets won't be available on HS2 with existing flexibility or effectively as good as it? I can't imagine charging what will probably be tens of thousands of pounds per year (given that Manchester to London's already close to £19,000) without being able to use said season tickets at any time.