Logically, paying benefits to those who don't need it runs a risk of someone "falling through the cracks" but equally logically you'd pay disability benefit to everyone in case someone's disability didn't quite meet the criteria. (Whether the criteria are appropriate is a separate issue, especially if assessors of ability are incentivised to reject applications.) In this case, the "crack" appears to be the nonsense that someone who qualifies for the WFP gets that benefit, but someone just over the threshold gets nothing, so ends up with much less than someone just below the threshold.
Re the rates, I thought it was instructive that Michael Heseltine - who benefitted massively (four big houses) was the first "big beast" to pull the plug on the poll tax (voted for it in Scotland in 1989 but opposed it in England in 1990).