Pi Calculus: Restriction necessary for molecular (atomic) action?
Your solution would work only if you consider $P$, $Q$ and $R$ in parallel as a "closed system", i.e. a system which is not supposed to be composed and interact with an external environment but just to perform internal reactions.

In the context of your quotation, the translation of a polyadic action into a monadic one is supposed to be compositional and to yield a bisimilar labelled semantics.
The role of the restriction is to avoid exposing the actions on $w$ to the environment: what happens if the environment performs actions on the public channel in your solution? 
For instance, consider the process $S$
$$
   S = w(y).S'
$$
If put in parallel with your version of $P$, $Q$ and $R$
it can "steal" the first/second component of the couple from $Q$ or $R$.
This is because you do not need to receive $w$ on $x$ to be able to use it if $w$ is a public name, while that is the only way to acquire it when it is restricted.