Fragility does damage every time a condition is applied, this can be over 30 damage for each applied condition if applied by a pumped mesmer (36 max.)
Fragility also seems to deal damage when a condition is reapplied, for example using deep wound on a target that already has deep wound on it. I am fairly sure this does occur.
The problem is it is difficult to tell when an innate attack reapplies a condition and this can potentially be very powerful. For example if a partner has apply poison an tiger's fury. The problem is the frag damage never shows when an innate property of an attack triggers fragility. Can anyone verify if the frag damage is trigger every time a condition is reapplied by mark of rodgort or apply poison? If so it has some very powerful potential. For example a team arenas team using mark of rodgort, fragility, apply poison, and fire weapons gaining large bonuses of damage on each hit. I have tried playing with this in Competitive arenas using mark of rodgort and it does seem to make a difference, but this is difficult to tell.