Removing material from the valve stem makes it weaker. If you remove too much material and it becomes too weak, the valve breaks off and typically wedges itself between the piston and the head, causing the car's power to decrease by 100 percent and dollars to rapidly leave your wallet.
On a race engine which gets torn down frequently, a few extra hp worth of flow might be worth the price of making your valves a inspect/replace item. On a street car, you aren't going to be frequently inspecting the valves, so you really need to err on the side of caution to avoid expensive catastrophes.
One other consideration with engine setups for the street is that you aren't constrained by the rules of a racing class, so you can get extra head flow from forced induction and nitrous on a street car rather than trying to squeeze power from a rules compliant engine. This is why you see spec miata guys paying 20k USD for an engine that makes makes 150 horsepower while the street guys are throwing together reliable 250+whp builds for under 5k USD. It isn't that making 150hp from a miata engine is hard, it's that it's hard to do without breaking the rules of the racing series.
Keep in mind that there is no guarantee that undercutting a valve gives any power at all, since flow through the head is determined by a LOT of variables, the size and shape of the valves being only a small part of the big picture.