When would it be advantageous not apply Training Ground's cost reduction?
You have a Glissa Sunseeker in play, and your opponent has an artifact with CMC {1}. You have 1 untapped Lotus Vale and no other untapped lands or mana in your mana pool.

You want to activate an ability on a creature that normally costs {3} (like that on Walking Wall), and you also want to use Glissa to destroy your opponent's artifact.

After getting 3 mana from Lotus Vale, you need to spend exactly 2 mana in order to have {1} remaining, so that you can destroy your opponent's artifact. So you choose to only reduce the cost if your ability by {1} instead of reducing it by {2}.