If you took armor yellows and MR seals, which was a staple in a huge majority of situations, you would have 9 armor and 12 MR. Now, if you take the armor and MR defensive stat runes, you get 5 and 6, respectively.
First point to remember: When Runes Reforged hit, every champion got compensation for their seals (mostly, they got said 9 armor, except for mages, who got their 12 HP/level they had as seals). Also, AD champions got their marks incorporated into their kits, as well.
The main runes not incorporated were the glyphs and, in a lot of cases, the quints - and while the path bonuses were supposed to care of the power loss from quints, the 12-27 MR from glyphs most people used just vanished.
So, if you go double MR, you get those 12 MR back - and with the remaining offensive slot, the power of 2-3 quints (2 AP quints gave 10 AP, whereas 3 AD quints gave about 6.9 AD). Basically, with this update, you now have virtually all the stats from the common rune choices back.
This doesn't seem so bad, except that we have access to a lot more damage and penetration now. With things like keystones like dark harvest and electrocute, coupled with a ton of armor/magic penetration items, the 5 armor and 6 MR is not only a joke, it's almost worthless. Even in the early game, especially considering the fact that you can get 12 AD or 20 AP at level 1 with adaptive force.
Early game, AD and AP were always stronger stats than resistances (or resistance penetration). This is due to the fact that AD/AP are primary stats and resistances are multiplicators for another primary stat (Health). From that PoV, I agree with you that the HP choice is underpowered.
However, another factor that plays into the relative power of resistances vs. offensive stats is the presence of base damage.
However, if you look at the value of 10 AP and 6 MR, you'll find that they are, in fact, comparable:
A 250 base damage, 0.8 AP ratio ability against a 0 MR/6 MR target.
With 0 AP, the ability does 250/235.8 damage (94.3%)
With 10 AP, the ability does 258/243.4 damage (94.3%)
10 AP increases this ability's damage by 3.2%/3.2%.
That same ability against a more realistic 36/42 MR target.
With 0 AP, the ability does 183.8/176.1 damage (95.8%)
With 10 AP, the ability does 189.7/181.7 damage (95.8%)
10 AP increases this ability's damage by 3.2%/3.2%.
That same ability with more AP (200).
With 200 AP, the ability does 301.5/288.7 damage (95.8%)
With 210 AP, the ability does 307.4/294.4 damage (95.8%)
An additional 10 AP increases the ability's damage by 2.0%/2.0%.
These examplary numbers show the general trends behind these values:
For someone who primarily builds AD or AP, getting the resistances will make more of an impact than getting the additional AD or AP in the majority of the game (all the time except at the very early levels were the base damages are really low).
For someone who primarily builds defense, in contrast, the AD/AP will mean more.
