I disagree... first of all... he is a 3star for the level, but 5star for some other... so the auction can be hard to win in every case. And a "slow" 5-stars is not so far expensive than a "fast" 3-star.
Second point... in every case, even if the 5star is slow to gain SP it will be always stronger than the 3star, given the same training, at least for the whole season, so when then it's time to change him in every case :-)
In any case, everyone uses the strategy he preffers, just it looks strange to me.