I feel many people feel that HS2 may not be fully integrated into the National Rail Network, meaning that it has limited benefits for most people. Also whilst it may benefit the economies of London, Birmingham, Manchester, Leeds etc... these cities have been doing relatively well in recent decades how well will it help those cities and regions that actually need the help like Bradford, Liverpool, Nottingham, Hull, Middlesbrough, Sunderland, Cumbria, Scotland, Wales etc...
That's a big list of places - I don't recall people complaining that re-opening something like the Woodhead won't be of benefit to people in Wales and using that as a stick to beat it with.
But, since you asked, at the moment lines like the ECML are full due to the need to provide fast services from London to Leeds/ Newcastle/ Edinburgh. That means services currently running non-stop through places like Peterborough/ Doncaster and scarce paths being prioritised for these big cities at the expense of Hull/ Middlesbrough/ Sunderland/ Bradford.
If you take the fastest services off the southern end of the ECML then you free up a number of paths that could be used for services like London to Hull/ Middlesbrough/ Sunderland/ Bradford, you free up capacity for more services to stop at Peterborough/ Doncaster, you free up paths that could be used for services from Stansted/ Cambridge/ Norwich to Yorkshire.
Take the fastest services off the existing route and you create space for "secondary" places to gain direct services.
Same goes for the WCML and MML, where taking some of the longer distance services off existing routes frees up seats for shorter distance passengers, frees up capacity for more services to stop at intermediate stations, allows more paths to run (e.g. if everything left on the WCML runs at 110mph max then you can fit in more trains than the current 110/125 timetable).
Liverpool, Cumbria and "Scotland" will get direct services via HS2, so they benefit.
Nottingham
shire will have the option of a High Speed service at Toton (convenient for those on the University side of Nottingham and those people who would have to drive into central Nottingham to get the current EMT services) - it won't be suitable for every passenger but it provides an alternative. People on here love the idea of competition when it means cheaper tickets on a 350 taking an hour longer to get up the WCML (compared to a 390) yet seem to dislike Toton because it won't be 100% suitable for 100% of passengers currently using the station in central Nottingham.
Wales? It won't benefit much (other than faster Holyhead - London journeys possible with a change at Crewe). But then will Yorkshire benefit from GWML electrification to Cardiff? Does that mean that they shouldn't have wired the GWML though? Or maybe we could look at projects in terms of who they benefit rather than dismissing them because they don't solve every problem for everybody.
It's a train service. Mass transportation, moving large volumes of people more efficiently than cars/ buses/ planes over the kind of distances HS2 is being built for. So what if it doesn't rebalance the entire UK economy and provide an end to urban squalor in post-industrial towns - there are other government policies better suited to that.
That's why I view HS2 as a wrongheaded solution to a non-problem. The cash could be better spent on improvement projects at either end that would not only have similar effects in reducing overall journey times but benefit local journeys too
Such as what? Better light rail/ local trains within each city/urban area (rather than anything inter-city)?