Mass Effect 3.7: "That was for Thane"

[Psyren]

I do not see how the Krogan Sentinel in any way is better at defensive play than the Krogan Soldier. Tech Armor and Fortification are essentially identical.

Incorrect - tech armor has slightly stronger DR (+15% I believe) whereas fortification passively adds melee damage instead. In addition, tech armor's detonate is more useful defensively (area stun) than fortification's (boxing gloves.)

Same deal for the other such characters: how is the Vorcha Sentinel better at defense than either of its counterparts? Or the Batarian? When they all have the same defensive powers at their disposal, the same Fitness or fitness-analogue, and their other abilities aren't defensive, how can that be?

Zevox

The Vorcha Soldier is more of a caster (especially now that it can detonate its own fire explosions.) It benefits more from lower cooldowns than the Sentinel, so it will want high passive as well, and a light layout - this will push it away from either the regen ability or fitness. The Sentinel meanwhile, with its grenades, can dump its only cooldown (Flamer) and perform better with a high-fitness build and heavy layout, becoming a highly mobile and durable skirmisher - akin to a Krogan that can dodge.

For the Batarians, this is even easier - Submission Net is defensive because it locks down unarmored foes, allowing you to e.g. freeze a phantom and revive the ally it was camping. Ballistic Blades meanwhile are purely offensive. Both get Blade Armor, but the Soldier will find the melee evolutions slightly more attractive since Ballistic Blades will want it to be up close anyway.