It sucks for device manufacturers because it is costly to build.
It sucks for consumers because the manufacturers have an incentive to drop support for any device sold asap WRT software updates.
It sucks for developers because custom components are not standard across devices.
The only point in favour of custom skins is that it theoretically allows manufacturers to differentiate and as a result offers more choice to end users.
In reality though, I suspect very few end users to pick their Android based on the custom skin it comes with, and I'm sure many would be pissed if they realized that custom skin is the reason for them not getting free ota upgrades to something they gave a nice chunk of cash for.