Yeah, but it's a very, very tenuous link.
KP was on a tour of Australia with England in 2007. England were hopeless, and Australia were one of the best teams of all time. Australia mauled England so violently it slightly hurt my eyes, but KP scored a fair few runs. Those runs were ****ing worthless, because England wouldn't even have won had the entire Australian team had their legs amputated before the first test.
The next winter England toured New Zealand, and in a hard-fought series, came back from 1-0 down to win 2-1. KP again scored well, with one particularly crucial, series-changing century on the first day of the (IIRC) second test, without which England might have lost the series. Those runs were worth much more to England than his contribution in 2007.
It's tenuous. How much runs are worth in the context of a match is only very, very loosely linked with the quality of opposition. So my question is, why are the runs against good attacks so invariably considered the sign of a better player? I don't think the reason you've given is the true one.
I am not denying that KP's runs in NZ probably did more for England's cause than his runs in Australia, although as has been pointed out, this is really in the benefit of hindsight and few players really know how valuable their runs are at the time when they are scoring them.
However, I am not sure I understand the logic of how scoring more 'worthwhile' runs for your team automatically make you a good player. Scoring runs against McGrath, Warne, Clark and Lee when almost none of your peers are able to do so is a sign of being a class above the rest regardless of how badly your team loses. Scoring runs against O'Brien, Mills, Oram and Vettori is great in that it may help your team win, however there is no logic behind saying that a player is more skillful for scoring runs against that attack than he is for scoring against McGrath and co simply because it helped his team win. By this logic, Andy Flower would have to have been a no-hoper given the number of times his knocks actually had any significance to his team's cause. Almost all of his runs were 'worthless'.
When rating a player, it is implicit sign of skill that runs scored against stronger attacks mean more than runs scored against poorer attacks because quite frankly scoring runs against stronger attacks requires a skill that few possess. This is analogous to scoring runs in county cricket vs scoring runs in international cricket.
I am not saying ignore all statistics against the weaker teams, Im just saying look at it in context. Someone like Yousuf can score billions of runs against the bottom four rated test teams, but what will truly end up separating him from the upper echelon of players is the fact that he couldn't buy a score against a bowling attack that wasn't in the bottom four of the test championship table. Couldnt care less how many games he won for Pakistan against WI, NZ, India and Bangladesh.