The arguments are actually about two issues. Long term supply-side incentives and income distribution. The first is much more straightforward to explain than the second.
One of the biggest problems faced by those out of work is the difference between in-work and out-of-work income. When the difference between the two is small the incentive to take a job is less and may prompt some people to remain on benefits rather than take a job. This is known as the unemployment trap.
During the period since 2008 the incomes of those in work have risen only slowly and at less than inflation, and for some nominal wages fallen. But benefits have gone up in line with inflation, which in some years has been well above target. So the difference between in and out-of-work income has narrowed. The government argue that the cap on benefits will restore the balance and raise incentives to work.
The case on income distribution can be argued both ways. Those on benefits are the least well off in the UK and it has been long established that they will be guaranteed an income that allows them to at least take a part in the normal activities of society. Therefore raising benefits by less than inflation represents a real cut in their incomes and breaks this social contract.
However against this the case can be made that the faster rise in benefits than in wages has led to an unintended change in the UK's distribution of income. While those on benefits will be worse off through this measure they will simply join the rest of society in the experience of recession. The distribution of income will be restored to the one that existed before.
There is no doubt that the distribution of income is the macroeconomic target which gets the lowest priority. The question really is do we want to prioritise long-run efficiency or compassion. No easy choice.