This is exactly what I've been doing as of late. My point-earning (via AmEx Explorer/Platinum Edge and CommBank Smart/Ultimate) is not as rapid as I'd like (no MS or biz trx to funnel through '000s), so I've resorted to waiting for promos for either Velocity (usually 40% off) or Aeroplan (usually 80% bonus) to redeem SQ J seats. Works out much cheaper than buying cash J fares outright.
Of course doing it this way does require careful planning (usually understanding the release timelines of each carrier's J seats and ensuring your account is loaded up with the points prior to these releases), and a comparison of cash J fare purchases vs the point purchases, but it's a great way to get discounted J seats without stumping up the full cash fare.
I've done the maths (if the username doesn't give it away), and for the routes I've redeemed for, especially for 2 pax, I've been able to wrangle a ~55% discount (Business Standard fare = $11,174 vs. Points + Tax = $5,008). This is substantial, just requires good timing, good planning and good luck!
So no, it's not too good to be true!