ye it is like this, u can use sup absorption for example on your chest, becuz you get most hits there, but some people prefer nrg or health, and on chest nrg and health bonusses are the biggest so they prefer it. you could use like sup absorption on ur head where you only have +1 nrg bonus, but on ur head u almost never get hit.
Superior absorption is global, so wearing the rune on your feet still gives you a -3 damage if you are hit on your chest.
From the Wiki:
Runes of Absorption reduce damage by 1-3 depending on potency. Minor Runes of Absorption reduce physical damage by 1, Major by 2, and Superior by 3. Runes of Absorption only reduce damage from physical damage and have no effect on spells or other sources of damage. Runes of Absorption do not stack with each other. This effect is global.
the minus 3 on physical damage is usually not very usefull, and people prefer other runes that reduce blindness, and/or crippled. For PvP it is more important to think of a offensive role as a warrior, then a defensive tank (like in PvE).
Some people use still a sup absorption when they go as part of a split team, but whether this is common that I cannot tell u.
The great thing about a rune of sup absorption is that it doesnt cost you health like other sup runes, yeh it can be useful in pvp such as a war in a gank team that will be fighting against physicl dmg dealers like rangers can help, 3 damage doesnt seem like a lot but does add up and it does help.