Why didn't they scale it from both ends, so we'd have round numbers at 60 and 70? Since you spend 90% of your game time at max level, it would seem to make sense, no?

Blizzard for some reason has never bothered to give us nice round numbers for about any formula so far (consider Attack Power and Crit and Dodge numbers have often not been integers for various classes)

I made a thread about it here:
http://forums.elitistjerks.com/viewtopic.php?id=9446

I think the actual values were chosen for convenience for the itemization team. For the most part they match very closely with the StatMods reverse engineered by Hyzenthlei at level 60.

+8 Hit Rating (Spells), 1% Chance to Hit (Spells), StatMod: 8.00
+14 Crit Rating (Spells), 1% Chance to Crit (Spells), StatMod: 14.00
+10 Hit Rating (Melee), 1% Chance to Hit (Melee), StatMod: 10.00
+14 Crit Rating (Melee), 1% Chance to Crit (Melee), StatMod: 14.00
+13 Block Rating, 3% Chance to Block, StatMod: 4.33 for each % chance to block (Which equals 13.00 for 3%)
+12 Dodge Rating, 1% Chance to Dodge, StatMod: 12.00
+20 Parry Rating, 1% Chance to Parry, StatMod: 20.00
+1.5 Defense Rating, +1 Defense Skill, StatMod: 1.50
+2.5 Weapon Skill Rating, +1 Weapon Skill, StatMod: 3.00 for daggers, 2.30 for others.

Late Edit:

That's also why the few rare existing low level items with some of those stats are pretty skewed at the level you get them. If they weren't, they'd be heavily underitemized and would need other stats to compensate.

But that can't be true, since each thing's worth varies by class, spec, and your other gear.

For example, as a warlock destruction specc'd, %spell crit is more valuable than %spell hit for nuke dps, but less for dots and life tap. But an affliction spec lock %spell crit is always worth less than %spell hit.

1 rating of one type is not = 1 rating of another type.

Not to mention that, for a druid tank, + parry/block are utterly worthless, +defense is marginal, and +dodge is what they really want for mitigation.

There's plenty of similar situations. A blanket statement of "1%x is always greater than 1%y" is sheer folly. As mentioned above, the ratings seem to be chosen to correspond to already-existing itemization values.

But those item values on the other hand are based on the actual ingame value of the stat(at least that would make sense).

These values are just the rule, of course there are exceptions as already mentioned.
Quite frankly I like this insight into blizzards way of itemization.

So 1 rating really is always "worth" 1 rating but only in terms of itemization value.

Blizzard really doesn't care what you believe a stat to be worth. Remember when MC first came out and most guilds were wiping in UBRS with 20 people, everyone though Spi was completely useless and Int was the only worthwhile stat for casters (kinda was, due to the way FSR worked back then) now I'd be more than happy to trade almost all my Int for more Spi/Mp5.

Chicken, where did you get those numbers? http://www.wow.lv/content.php?article.44 Is the only source I have for the formulae, and it disagrees with the numbers you have multiple places.

Mine were from WoWWiki, not the best source of information I know. This page: http://www.wowwiki.com/Formulas:Item_Values

They do mention these were shown by Blizzard at BlizCon however :)

Remember that the only ones Blizzard mentioned were crit (14) and dodge (12). The rest were derived from comparing Hyzenthlei's results to what Blizzard stated.