Sure consistency is great but it doesn't trump everything else. If that were the case we would still be using Nokia style interfaces from the 90s.
Samsung might benefit from adopting the Android 4.0 UI. If they did, assuming other vendors do the same, there would be a more unified UI throughout all Android smartphones, not unlike iOS. And people complain about Android fragmentation...
Why would a handset manufacturer want the UI on their devices to look like those on every other? The thinking here is to differentiate as much as possible - so TouchWiz helps by making the Samsung devices look different than other models you see in a store. This is the "value add" over stock Android.
Samsung might benefit from adopting the Android 4.0 UI. If they did, assuming other vendors do the same, there would be a more unified UI throughout all Android smartphones, not unlike iOS. And people complain about Android fragmentation...