The pitch for speed mantra developed back in the days when most approaches were glide approaches and so power was not in the equation anyway. If you got low, you had to fix up the flight path ('cheat') with a trickle of power. This still applies for glide approaches.
But if you are being taught powered approaches, the more modern thinking is power for speed, pitch for flight path. In reality - as a previous post suggests - you will probably find that with experience you do a bit of both without even thinking about it.
If you ever get to higher performance aircraft, your instructor will insist on power to control speed on the approach. So it seems your current instructor is preparing you for better things.