I see @marcus answered while I was typing this, but I wanted to add a few things:
The optoisolator protects you and your Spark core from the high-voltage. Your Spark has to get 3.3V or 5V from somewhere and isolating that from the high-voltage AC is a good idea. If you do as you suggest, the entire core and everything connected to it has to potential to be at high-voltage and very dangerous. Please don’t do this. You should take extreme care to make sure all of the high-voltage wiring is physically separate from the low-voltage Spark side with the optoisolators being the only things that cross the domains. Even then if you are working with 220V, you may need to have an air gap milled in your circuit board to improve isolation.
Unless you are dimming a light bulb, you want to switch AC at the zero crossing to avoid problems with the load seeing partial cycles and high inrush currents and to avoid generating RF interference. Triacs are funny devices that require special handling on their gate input, a function normally provided by diac either inside the triac or a separate device. The zero-crossing detector in optoisolator removes the need for a separate diac. Here is a good description I found quickly by Googling:
I don’t understand your third point: the optoisolator zero crossing detector is on the high-voltage side and is a good thing to have if you don’t want to dim a lamp.
The optoisolator is a small low-power device that can control milliamps of current, not the many amps required for a typically load.
Finally, maybe you should just buy a solid-state relay. They are essentially all the components you have in your diagram in a nice package for not too much money. You do have to heat sink them (like you would your triac) for big loads. A 240V 10A rated SSR is under US $3 on ebay and a 40A SSR is less than $10 at Sparkfun.com.
Why not just buy it? That will be much easier in the long run.