Node inheritance causes a large number of complications and ambiguities that make understanding puppet code difficult. There is very little reason to use it since much more effective reuse can be achieved using classes and defined types.
On the next 3 series puppet that is using the current parser: the system must issue a deprecation warning when it encounters "inherits" in a node statement.
For the future parser (3 series and in 4) inherits on a node is an error.
UPDATE: The final decision is to make 3.7 (current) issue a warning, and that 3.7 (future) raises an error. This to make the 3.7 future as close as possible to what will be released in 4.0.