On my early examination of the durability tree, I realized that it might be a trap. My initial inclination, upon seeing the current iteration of the durability tree was to call a twenty-six node investment a heavy but worthwhile investment to make mechs extra-tanky. For a sixty-five ton mech--and I was initially theorycrafting skill tree builds for my Linebacker--the bonuses for that investment are 13.6% armor and 25.6% structure, which sounds like a lot.
There's a couple of important questions that need to be asked about those percentages, though: How many absolute hitpoints do those percentages translate into, and what do those bonus HP mean in practical terms?
The first question is simple math, provided in the table below:
Mass CT Ar/St 26 Ar/St Nodes CT Bonus HP 20t 24/12 20.8%/32.8% 4.992/3.936 30t 40/20 19.2%/31.2% 7.680/6.240 40t 48/24 17.6%/29.6% 8.448/7.104 50t 64/32 16.0%/28.0% 10.24/8.960 55t 72/36 15.2%/27.2% 10.94/9.792 20.73 bonus HP 60t 80/40 14.4%/26.4% 11.52/10.56 22.08 bonus HP <- Peak armor bonus 65t 84/42 13.6%/25.6% 11.42/10.75 22.17 bonus HP 70t 88/44 12.8%/24.8% 11.26/10.91 22.17 bonus HP 80t 100/50 11.2%/23.2% 11.20/11.60 22.80 bonus HP 85t 108/54 10.4%/22.4% 11.23/12.10 23.33 bonus HP | 185.33 total HP 90t 116/58 9.6%/21.6% 11.14/12.53 23.67 bonus HP | 197.67 total HP <- Peak structure bonus & peak bonus HP 95t 120/60 8.8%/20.8% 10.56/12.48 23.04 bonus HP | 203.04 total HP 100t 124/62 8.0%/20.0% 9.92/12.40 22.32 bonus HP | 208.32 total HP <- Peak total HP
As a function of tonnage, the bonus HP explodes from the nine HP that an unquirked twenty-ton mech gets to its center torso, until it nearly levels off at about twenty-two bonus HP around sixty tons. At that point, the armor bonus actually starts to get smaller, but the structure bonus continues to grow fast enough to make up the difference, until the total bonus HP peaks at about twenty-four for a ninety-ton mech, after which point the structure bonus begins to decline alongside the armor bonus.
In practical terms, a dual-Gauss hit eats up those bonus HP. A dual-(ER)PPC hit steals almost all of those bonus HP. Four IS medium pulse lasers burn through those bonus HP in their 0.6-second burn time. In other words, twenty-four bonus HP basically buys you a single extra hit from weapons that don't badly spread their damage.
Is a single hit worth spending a quarter of your skill nodes? Ultimately, that's for you to decide, but I'm inclined to say, "No." Saving the twenty-six nodes that would otherwise be put into the durability tree allows you to hunt more heat gen nodes in the weapon tree and more torso speed nodes in the mobility tree. Faster twist speed will allow you to better place incoming damage where you want it to hit, and lower heat generation will help you fire the extra shot you need to cut through your target's bonus HP.
Caveat: My interest was in the general case. The effect of quirk HP and the bonus HP that the skill tree provides to locations besides the center torso I leave as an exercise for the reader. Most kills are CT kills, and most mechs don't have (large) durability quirks, so doing the math for an unquirked CT felt like the most productive use of my time. You'll have to do the math yourself, if you want to know if dozens of durability nodes are worthwhile for the special case of an XL Atlas.