You can pick and choose specific figures and rates, but regardless, what's wrong with simply merging NI and income tax?
Absolutely nothing, I'm with you 100%. Politicians have historically avoided it as they've had a habit of decreasing income tax rates to 'keep to their promises' whilst increasing NI to pay for it.
The only slight problem is in relation to pensioners and child earners because you only pay NI when between the ages of 16-65. There should be an easy way of dealing with that though.