What's the point of always "changing" the tomestones?

[Niwashi]

it would have been nicer to leave each tomes for each major patch (2.x, 3.x, 4.x) separate so that they could separate the dungeon and roulette rewards so people don't spam MSQ/POTD for poetics for HW gear.

Consider a few possibilities:

1) Each expansion has its own tomes, and rewards are based on the content you're running: Would there be enough demand for Poetics to keep the queues moving for ARR dungeons? Would there be enough demand for Scripture to keep the queues moving for HW dungeons? If one or the other type of old tomestones aren't considered desirable enough to level 70 players, it could be hard to ensure all the MSQ required instances are accessible to new players.

2) Each expansion has its own tomes, while rewards are in the tomes suited to your current level: It would get level 70 players into old dungeons well enough, so avoids the problem of dead content roadblocks, but what about when those players actually want the ARR/HW tome gear for glamour? They'd need some other way of getting it.

3) Each expansion has its own tomes, and rewards include all of them up to your current level: Extrapolate out a couple more expansions into the future, and 6.0 players would be getting tomes for 2.0, 3.0, 4.0, 5.0, and either one or maybe a couple tiers of 6.0 tomes at once for each dungeon run. Do they really deserve five or six times the rewards that an on-level player would get just because the game doesn't know which gear they're trying to grind for?

The one they ended up choosing is kind of like option 3 there, except that by combining everything below the current expansion into one, they're keeping the total rewards for a run at a more reasonable level.