With Precision, you have [ 3 / 5 / 6 / 8 ] mpen at levels [ 1 / 6 / 11 / 16 ].
For Piercing Thoughts, which gives you flat 7% at all ranks, to be more effective than Precision, your target must have more than [ 43 / 71 / 86 / 114 ] MR.
Most champions either have 30 or 32 base MR, and some champions have scaling MR while some do not. Those with scaling MR will have [ 32 / 38 / 46 / 52 ] base MR. We can see immediately that Precision out performs Piercing Thoughts for targets that do not have any bonus MR. If someone is running flat MR blues, they will have +12 MR (42 or [ 44 / 50 / 58 / 64] MR). In this case Piercing Thoughts is a superior choice from level 2 onwards. If someone is running scaling MR blues, they will have [ 31 / 39 / 46 / 53 ] or [ 33 / 47 / 62 / 75 ] MR, and Piercing Thoughts is still a superior choice.
This means if you mostly care about blowing up squishies, who typically don't have an MR item for most of the game, Piercing Thoughts is always better (and so is thunderlords, if you have the proc up).
In order for Piercing Thoughts to be more effective, your target needs to acquire:
[ 13 / 41 / 56 / 84] MR if they are not using MR runes and don't have scaling MR.
[ 11 / 33 / 40 / 62] MR if they are not using MR runes and have scaling MR.
[ 1 / 29 / 44 / 72] MR if they are using flat MR blues and don't have scaling MR.
[ 0 / 21 / 36 / 50] MR if they are using flat MR blues and have scaling MR.
[ 12 / 32 / 40 / 61] MR if they are using scaling MR blues and don't have scaling MR.
[ 10 / 24 / 24 / 39] MR if they are using scaling MR blues and have scaling MR.
Most mages don't have scaling MR (generally, the ones that do are melee range). Buying a 
, which gives +25 MR, doesn't cut it after level 6 with any rune configuration. If the only MR item they're ever going to finish is a 
, then Piercing Thoughts will never be more effective. If they're building a 
for 
, (+40 and +50 MR) then Piercing Thoughts might be better until around level 11, and probably won't at the end of the game.
ADCs might buy a 
, 
, or 
(+35, +70, +50 MR), and all have flat MR. If they run scaling MR blues, only the banshees veil will make your Precision worthwhile.
AD Assassins mostly have scaling MR, but won't pick up much more than a for most of the game. If they run scaling blues, then Piercing Thoughts is better once they finish .
If you buy 
the numbers don't change because it doesn't affect how much penetration Piercing Thoughts gives you; it's calculated before flat penetration. You can still compare that number to the flat from Precision.
Keep in mind that many supports will pick up 
, which gives +10 MR to the other team, especially if you're doing well.
The bottom line is that, for blowing up ADCs, fighting most matchups in your lane, or fighting any jungler that doesn't rush MR tank items, Precision is going to be better for most, if not the entire, game. Combine that with the fact that Thunderlords outperforms Deathfire Touch on most champions for the entire game and all champions for the majority of your lane phase, 18 in the Cunning tree is just plain better.
The problem isn't just that Deathfire Touch is underwhelming for most of the game (you need something like +3AP per level just to overcome the target's natural HP regeneration) on most champions, it's that Precision is also a better supporting mastery.
Yeah, DFT procs on every spell, but you need massive amounts of AP for hitting three spells with DFT to break even with one thunderlords proc. You can also proc thunderlords with auto attacks. Even champions on whom DFT is more efficient will do better damage early with thunderlords, and the early game is more important than ever.
TL;DR the numbers don't back up what you're saying.
