I disagree.
If I were planning a journey from, say, Buxton to London on the 15 May, I could today buy a through Advance ticket for £26, only £2 more than from Stockport to Euston. But for travel the following week journey planners cannot yet offer through services, because the Northern timetable is not loaded. I would have the choice of either:
a) waiting another 4 or 5 weeks to make the booking, by which time the Virgin Advance fare will no doubt have increased substantially (if still available at all), or
b) making a £24 Advance booking now from Stockport to Euston, but then later paying the additional Northern Anytime fare from Buxton to Stockport of £10.50. This would also mean taking the risk of having to buy an additional Virgin walk-up ticket, if the Northern train were delayed and I missed the connection at Stockport.
If Northern had already loaded a provisional timetable, I would be able to save £8.50 on that journey. The timing of the Buxton - Stockport leg might change a bit between now and May, but certainly not enough to make the journey impossible.