...You're acting like a net damage loss (generating Adderstings) is a necessary win condition. Toxicon does not make up for the damage lost to shielding. It's merely a small, partial refund that allows for some additional mobility or AoE even while forced out of melee range.
If your ally needed the shield that just happens then to give you some Addersting on its consumption, well... your ally needed the stronger SCH shield in that circumstance even more; you've wasted most of GCD of damage-dealing... to reduce your ally's shielding.
You do not need to shield spam for Adderstings. You do not, generally speaking, need Addersting. You certainly do not need a net waste of your DPS nor to waste your SCH's more powerful CD-generated shields to build up Adderstings.