According to e.g. the CEO of Citibank [1] and the mayor of London [2], immigration reform would really benefit the UK. The OECD suggests that there's no reason not to [3].
However, it's more complicated than that. The current policy is actively deterring young, skilled people from developing their skills and connections in the UK, which is a great concern for the House of Lords Science and Technology Committee [4] - who clearly understands that immigration is a long-term process, and governments can't treat people like sheep if they hope to attract talent.
If you were 30, about to start a family, would you immigrate knowing that you're not allowed to make money from tinkering on a side project for the next 5/6 years? And that after that time you wouldn't even be sure if you and your family could remain in the UK? It's a tough decision. And yet the best of the best are exactly in that position -- the people who tinker on the side are exactly the people who become the best, not the ones doing a 9-5 day job.
The irritating thing is that the policies were great prior to 2010, until Cameron decided to make a policy of reducing immigration to levels that are simply impossible ("tens of thousands"). His only way to control immigration is non-EU immigration, which is a very small percentage of actual UK immigration. It turns out that just a few days ago the government has finally started to acknowledge the sham [5], after years of criticism. Unfortunately in the meantime, people have to make very difficult life changing decisions in the face of this political drama.
[1] http://www.workpermit.com/news/2014-06-27/minister-uk-will-n...
[2] http://www.workpermit.com/news/2014-06-17/london-mayor-londo...
[3] http://www.workpermit.com/news/2014-05-23/migration-is-econo...
[4] http://www.workpermit.com/news/2014-04-14/lords-denounce-uks...
[5] http://www.workpermit.com/news/2014-09-17/deputy-pm-uk-has-s...