In the long run. For private renters anyway.
As things currently stand, private landlords are getting away with charging extortionate rents that few can afford because the shortfall is made up by Housing Benefit. So if benefits are to be capped, landlords will (hopefully) be forced to lower their rents to affordable levels or sell up if they can't find tenants that can and will pay hundreds of pounds a month. Either will be great for the average Joe IMO.