@Meddler: What are your thoughts on a damage skill that bypasses shields?

Shields, as we all know, are really handy for preventing damage, with the only options to getting past them being either to break them with damage or wait until they expire. But what if there was a Champion who had a skill that could completely IGNORE a shield and deal damage through it?

Here's a prototype skill to illustrate it (take with a grain of salt):

Your next basic attack deals bonus damage. X% of this damage is converted to true damage and ignores shields. Resets attack timer; applies on-hit effects.

So, for instance, say Janna shields a teammate for 400 damage. This skill deals 300 total, with 40% converted to true damage that ignores Janna's shield. The shield takes 180 damage (not taking resists into account), leaving it with 220 shield remaining while the Champion underneath takes 120 true damage.

Would this be neat for a future assassin to have, or would it break gameplay rules even if it was a mechanic that only one Champion has?