Think about it in terms of different classes instead of different games. Class A only has at-will powers, while class B has at-wills and powers that are regained after a short rest. If class A is to be balanced with class B, and powers that are regained after a short rest are better than at-will powers, then class A's at-will powers need to be better than class B's at-will powers. This makes class B better than class A for shorter, presumably easier encounters (when fewer at-wills are used), and worse than class A in longer, presumably tougher encounters (when more at-wills are used). Whether the difference is significant or not is another issue, but it certainly does look like there will be a difference.