hmm not sure why you say it's useless, "for integer vs % damage reduction, the flat comes first. This is good for if you are amp'd or decrep'd, since it means you get up to double the effectiveness out of flat DR. If you have % DR investment, these two stats are antisynergistic because the flat comes first, and the % mitigates a percentage of what is left over."
"Flat absorb is applied after resistance (very important), even though integer DR and MDR are applied before resistance. Being applied before resistance like in the case of MDR means that resistance is less impactful for overall mitigation, whereas with sorb, higher resist is very synergistic with it. 75% resist and 20 flat sorb means that the hit is taken down by 75%, 20 sorb is subtracted from the hit (unless the hit is already mitigated down to below 20 damage), and then 20 life is added back. If the hit is more than 40 damage after 75% res, you won't heal, but you will subtract 40 damage from the hit. If the hit is less than 40 damage after res, 30 for example, you subtract 20 (leaving 10 more damage to be dealt) and then heal for 20 (10 of which is lost to the remaining 10 damage) and you heal for 10 overall."
basically the more res % you have, the better flat integer absorb becomes as it is applied after resistance.
In GM pvp however there are rules to avoid exploiting the absorb mechanic to the point of nullifying the opponent's dmg!