Although it's entirely an academic discussion, I'll bite.

If you earned the 300 tomes a week from the time you started your sub, you'd be able to spam out i90 gear right as you hit 50. If it didn't start accumulating until you had a combat job at 50, and it didn't accumulate when you weren't subscribed, it wouldn't fundamentally alter gear accumulation but would not offer any advantage to those who play more often than those who do not, which is not generally how MMOs are designed.

The developers feel that a week is a long enough period to allow for a few hours of playing. I don't see why consistent players shouldn't be rewarded versus those who cannot play for a few weeks at a time. Given the upcoming content that will allow you to spam out i80 gear I don't think keeping some gear exclusive to those who are actually playing the game is unreasonable.