A good reason not to use an ordinary light dimmer in series with a power transformer is that if the Triac is not conducting symmetrically on both positive and negative swings of the AC sine wave, then a DC component will be created.
As a transformer primary usually has a low resistance, a significant DC current will flow causing heating of the winding as well as core saturation. This in turn reduces inductance of the winding.
It's this principle that is behind saturable reactors used for power control.
The best way to deal with the situation is to use a Variac, or a light dimmer designed for transformer loads, which will be designed not to cause a DC component, as well as incorporating the correct RC network to deal with inductive spikes created.
