I don't really see this as true, since Freezing Touch is a hold. There is a difference in durations to consider, but if we analyze the numbers I think we'll find that the problem is negligible.
Freezing Touch is a scale 10 Hold, Frozen Aura is a scale 20 sleep on Minions and a Scale 12 sleep on Lieutenants. Since the damage from Frozen Aura interrupts sleep, it's impossible to stack it on anything higher than a Lieutenant. On a level 50 Blaster, this translates to 11.92 seconds of Hold, 23.84 seconds of Sleep on Minions, and 14.3s of Sleep on Lieutenants. On Minions, yeah, you could be losing out on a decent amount of time. But they're Minions, I don't think we generally need to be spending holds on Minions to survive. On Lieutenants, you're only missing out on potentially 2.38 seconds of control. Take into account power activation times, the purple patch shrinking control durations and reducing the difference between the two powers, and the fact that it's far more valuable to slot for hold duration than sleep duration, and from where I stand, it's a non-issue.