waste. steroids replace your native testosterone output. at some point, your test levels may become so low, there's no test to boost, and regardless, its not at a high enough level to give you more gains.
the only reason i could think of a test booster being a good idea, is if you're one of the few (5-10% of people) that has no libido while on tren. unless you're one of those, there's no point. and even then, there's only so much a natty test booster will do for you till you get off cycle. IMO.