I won't argue that Nash on the Mavs would have equalled a le. Nor will I argue that Nash's inability to defend Parker adequately is not a big reason why the Suns have consistently struggled vs. the Spurs.
However, if the Suns system had emphasized defense a little more and they had a quality shot-blocking center, Parker may not drive to the rim as often and thus Nash would not have appeared quite so bad. It was certainly a combination of factors. Nash plays in mightily, but so to did the non-emphasis on defense within the system, and the lack of size and skill down low on defense.
Suns trade Marion for Camby and Mike Miller last year (which was actually rumored amongst those I know with ties to the Suns organization), then Nash, Bell, Miller, Amare and Camby with Barbosa, Hill, Diaw and Skinner off the bench and maybe Nash's defense isn't as big an issue.
Effin' Sarver axed this because of the salary it took on, but it worked financially, with maybe a filler or two, but neither was a big enough name to justify the added expense.