The extra survivability is then translated into damage output and damage reduction for the entire party, using a combination of To the Death and Bodyguard. Initially I thought Counterstrike and Walking Fortress may be useful, but in practice they are very rarey used. Ultimately I decided to stick with Mark of the Rift and replaced Walking Fortress with another active ability.
Are you overlooking the CDR benefits of Walking Fortress with Siege Breaker, as well as synergies with Cutting Words and Horn of Valor?
Using Walking Fortress should drop all of your CDs by 8 sec, including its own CD, assuming a single attacker hits you once per sec. More attackers would lead to more CDR. Therefore, >30% CDR should be easy to achieve with proper timing.
Imagine quickly burning all of your CDs, followed by taunting all enemies and using Walking Fortress to refresh CDs.
To illustrate the power of CDR, consider upgraded Horn of Valor - 35% damage increase, or 140% across the party. Its default uptime is 10/18 sec = 55.6%, which yields to an average damage increase of 19.4% per character/77.8% across the party. Increasing its average uptime by 30% leads to damage increases of 25.3% (individual)/101.1% (party). Furthermore, if Walking Fortress is popped right after Horn of Valor, it's possible to have the CD refreshed during Horn of Valor's 10 sec duration, allowing 20 sec of the buff to start each fight, so effective uptime can be increased considerably over average uptime for most encounters.
A similar case can be made for Cutting Words, which increases party damage by 20% on taunted targets (80% cumulative damage increase across the party). CDR for all taunting abilities results in substantial damage increases for the party. I suspect that Counterstrike benefits from Cutting Words, making it an effective party damage buff, rather than simply a defensive ability. Also, Counterstrike activated during Walking Fortress against a large group of enemies yields massive CDR.
To the Death and Bodyguard synergize greatly with CDR through substantially increased uptimes.
I won't go into depth on additional benefits and potential abuses of CDR, but in a nutshell, more ability spam = more DPS, more utility, better aggro management through more frequent stuns, better survivability, more CC, more cross-class combos, higher focus generation rate, etc.
Disclaimer: I haven't personally tested the above abilities and relied on the game's notoriously inaccurate descriptions. Assuming the abilities and their synergies work as advertised, Walking Fortress seems too good to pass up for any Champion. As for Countersrike, I'd like to see more data on focus generation and Mark of the Rift damage to properly evaluate the raw damage of Mark of the Rift vs. the multitude of benefits of Countersrike.