seen this tool before, so I google'd it and it looks >impressive. If I understand it correctly, its basically a high-end >biscuit-type plug joiner. What I don't get is why is the dust >extractor such an expensive option. Does it blow air into the mortise >and then vacuums it out as well? If you're asking about the dust collector, it's as high end as the Domino tool itself. Much quieter that your regular screaming beast, very effective dust collection, tool activated power on function, very effective dust filtering. Naturally, there are others will forgo the cost for a $98 two hundred decibel animal. All I can suggest is to try on of the Festool dust collectors on their thirty day return guarantee program and then decide. Also, Festool offers a package reduction price if you buy the Domino and the dust collector at the same time
replaces the need for a biscuit joiner. Some might disagree with that statement, but I'm not one of them.
and edge gluing boards and plywood. Making a door panel, cabinet side, use the plate joiner. Aligns and adds a little strength. Extra strength not needed since its edge gluing, which are strong enough with nothing extra. Domino probably does a fine job too. If you use one of the wide hole widths you can not worry about exactly where you put the holes. But the plate joiner is quicker, lighter, easier to use. Its main purpose is to edge join boards.
By your statements above you have probably not used a Domino. The plate jointer makes a curved single thickness slot. The domino makes multiple thickness slots and every bit as effective in alignment as a plate joiner where a plate joiner is adequate. Any thing beyoned that the Domino is by far superior. The plate joiner is absolutely no faster than a domino and set up is absolutely not easier and for all practical purposes they both are in the same weight range for the same style plate joiner.
So use a plate joiner when "good enough" is good enough. If you want better up. down. and side to side alignment, and much more strength, go with the Domino.
Your few examples you listed are about are about all the plate joiner is decent for with one more exception, it makes a good slot for a lock lever to rotate into. With the Domino what you have listed above is only the beginning of what a Domino is good for.