In the case OP asked for, this is completely right. If you know you gonna fight, you need tank.
But if you try to avoid fighting, don’t be too tanky. Say you have a ninja mining Venture or a Jita-running indu. If the venture can get away very quickly (agility, not speed!), it does not need tank, but your vigilance. If the Indu aligns faster than the gankers can pick and lock, it does not need any tank either. If you are not in Highsec, tank just lenghtens your struggle (for your weapons are poor or non-existant), but in most cases doesn’t prevent your ship from exploding in the end.
BUT if you know help is on its way (i.e. CONCORD or fleet mates), enough tank to survive until they arrive is very healthy.
Example: I tried to get into these Triglavian hold systems with small but very agile T1 ships. After I got through the gatecamp, I was able to finish all sites and quests I wanted, and there were absolutely no rivals. So an Atron was much “stronger” (=effective) than a Vexor or Dominix, because in spite of having only 10% or 1% of the tank, it survived the camp just by running through.