From your question I am not sure if you understand the way TRIAC's operate. You can only turn on a TRIAC you basically can't turn it off under control.

A TRIAC is a current control device. If you tried to turn on a TRIAC at the zero crossing point it will fail. This is because current flow keeps a TRIAC operating once it has a control pulse on the gate. Once current flow falls to zero the TRIAC will turn off until the gate next receives a control pulse.

This is also why TRIACS can't be used to control inductive loads such as motors without extra control circuits. In this case when current stops flowing into an inductor the inductor sends current flowing back in reverse until the stored energy is used up. If this current flow lasts longer then 10ms (for 50HZ supply) then the TRIAC doesn't shut off properly. (maybe less then 10ms depending on when TRIAC fires).

When using TRIAC for dimmer control they are used by delaying the time they are turned on after the Zero crossing point for each half cycle. The later the TRIAC is fired the less power that is let through to light making them dimmer. And vice versa.

You are right because you turn the TRIAC on part way through the power cycle there is inteference generated. This can be picked up as radio interference but also heard as mechanical buzzing in the lights / dimmers.

The link I have included is a good explantion of how dimmers and TRIACS work.

Depending who you talk to this is known as forward phase control.

The interference problem is why the next generation of dimmers are known as reverse phase control dimmers. They don't use TRIACS they use various forms of transistors that can be turned off as well as on under control.

So instead of turning on as the half cyle is increasing in level they turn off as the half cyle is decreasing. This means there is a lot less interfence generated. The only thing against them is the control circuitry is a lot more complicated whereas a basic TRIAC dimmer only needs about 5 simple components to work.

Another new system is to use Pulse Width Modulation ( Sine Wave dimmer) to produce a sinewave output whose voltage level may vary but is still a sinewave. Once again the control circuirty can be complicated.
Hope this helps you.

The link below is also a good set of links on dimmers

http://www.epanorama.net/documents/lights/lightdimmer.html