Running it down is not the only issue. Nadal's extreme spin also does a lot to force errors from his opponent. The number of errors still (and will always) exceeds the number of winners. Anything then you can add to your game to force an error is a bonus.
As I posted, last week I hit for two hours against a 5.0. My shots had much more spin on them and dropped much more sharply. The 5.0 had trouble timing them and really didn't adjust within the 2 hours. The increase in spin then forced errors from him.
Volkl C10 Volkl Tecnifibre NRG 17