I think some teachers have some form of Stockholm syndrome about levels.
Did you teach before levels? Were you able to assess and formulate a way forward. Levels were supposed to summarise assessment for each key stage. As has been correctly said, they weren't supposed to be bastardised and form targets and awful assumptions about linear progress in all subjects. Worse, they formed the growth of things like APP - which led to dozens of hours being spent levelling on different Assessment Foci every week in the core subjects (fortunately the materials were never distributed for all the foundation subjects as the people writing them got stuck, realising (apparently) that levels aren't for that - I think that was the start of the end [and then there was a change in government anyway])
That's up to schools to work out a valid form of assessment. I've already said I don't know what they're being replaced with - and I do have concerns about that. I'd much rather there was a ready made replacement. I know several Academy chains are working something out and will probably sell their models. There are already several different and better models on the market for maths than levels.
I understand some of my peers worries that the system of assessment won't be transferable between schools, but I genuinely think nothing is better than levels as they currently stand - they mitigate against progress, cap aspiration, and are an insult to teachers with subject knowledge. Daisy Christodoulou has collected a series of links with really exciting things that teachers and others are doing with the opportunity here:
http://thewingtoheaven.wordpress.com/2014/03/17/replacing-national-curriculum-levels/
Incidentally, Tom Sherrington in that list has just been appointed head of Highbury Grove, which will be excellent for them.
Also from that link, this paper:
http://onlinelibrary.wiley.com/doi/10.1111/j.1467-9817.1995.tb00063.x/abstract shows that students with a KS1 level 2 in reading had reading ages varying from 5-10. As if levels are helpful assessments!