Oddly the personal allowance has increased significantly in recent years from £6035 in 2008/9 to £10600 in 2014/15, so I'm not sure many of those on tax credits are paying a lot of it back in tax. One problem though is that NI thresholds have not increased at the same rate, and pay rates have tended to use the minimum wage as the industry norm in some sectors. But also many people have adapted their working patterns around tax credits, making sure they don't work beyond thepoint where they would start to lose tax credits, and some have adapted their lifestyle so it now covers things that were luxuries 5 years ago.
I agree that in an ideal world there should be no need for tax credits- the minimum wage and tax/NI allowances should be sufficient. The added complexity of tax credits also means many people don't claim . I just hope they are phased out/cut back in a less drastic way as at present many will lose money that they won't get back until a year later