My 2 cents.
As already stated repeatedly above, dry yeast is "prepared" to get cracking the moment it gets rehydrated. It's primed and good to go as it is, ergo it is unnecessary -and in fact downright detrimental to yeast performance to make a starter from a pack of dry yeast. Not sure if this applies to strains like saflager as well though.
As to "one should not"...that sounds a bit silly. I mean, you can easily rack a second beer on top of a yeast cake originating from a previous fermenation started with dry yeast. That's not the same thing as a starter but as close to it as I care to reckon. You could just as successfully make a starter from a single packet to arrive at a cell count which would be more suitable for (say) a big-ass imperial stout, or a 10 gallon batch instead of 5.
From my understanding, it'd still not be as efficient as simply ptiching more packets, but at the end of the day, the packets contain healthy yeast, which you can use either to brew or to propagate.