I wrote the NUS' policy on this proposal, which says that they're neutral on it (was difficult to consult students on), but against it if certain conditions aren't met.
There is a fair case for the system needing to change. It was designed during a time when very few students attended university, and clearly isn't great for upscaling to a time when 40-50% of school leavers do. Employers do say it's difficult to distinguish between candidates, particularly with the large number of 2:1s, it's not as well recognised internationally where GPAs are more the norm, and it has resulted in lots of graduate recruiters simply using it as a filter (must have 2:1, but what's the difference between 58 and 61% in real terms?). There is far more to university and 'employability' than marks, but they are certainly a factor.
The proposal which has been put forward sucks. It uses a scale of 0-4.25 unlike any other GPA in the world. Marking would be in grades (A+, A, D, F, etc) which are discrete, then converted to a GPA which is continuous. Particularly for STEM subjects, you're going to lose valuable data. Grades in one subject could correspond to something different in another, and similarly between universities, making it extremely difficult to compare them like for like, effectively removing a lot of the benefits of having a national standardised system with appropriate local flexibilities. It's highly likely that anything above say 80% would result in no better a GPA, as the top grade of A+ would represent say 78-100%, effectively punishing students who do extremely well in particular areas.
A psychometrician a few months back proposed to me that the only way to please people would be to get rid of the grades, GPAs, classifications, and just give people a mark of 0-100%. Whilst I tend to agree, it could have mental health connotations with students thinking every single mark counts, which it just about would.
There is something in this, changing the system, but the ill-thought and hashed together GPA proposal has been a joke.
Instead, I think they should first research students' motivations to learn, to better understand what the advantages and disadvantages of various marking and grading systems are, including the status quo, before proposing something like a US GPA given a UK twist. I imagine the conclusion will be that marking and grading isn't the best way to motivate students or to help employers find their ideal graduate employees.
To put it more formally: