Note that it is rounded up, so while 90 points of armor has threshold of 9, 91 points of armor has threshold of 10. Most anti-aero unit has AC/2 or AC/5 so you can live with 41 armor points on each location at least and only fear for the control rolls by struck by an attack, and if you want to withstand Clans Large Pulse Laser(10) you need at least 91 armor points. If you want to withstand a Gauss Rifle hit you need at least 141 points. 191 points or more lets you to withstand an Air-Defense Arrow or Air to Air Arrow missile.
Well, we all know that a struck aero unit is likely to crash, so unless you activate Advanced Atmoshperic Control Rolls(97.SO) that only need to have a control roll if the hit exceeds its damage threshold, a flying brick is less viable. Also because it is too cheesy to run the real brick with 30+tons of armor with this, if you really want to use the optional rule with the brick aero unit you better use it with Variable Damage Thresholds(117.SO), which counts the threshold by the current armor points rather than the default armor points. So your brick may sustain some hits without a control roll, but the sustained hits with heavy hitters makes you tremble(literally).
And I don't understand why an aerospace fighter can put up to 50% of its total mass with the standard armor. Is it a MBT of 21th century?