Point one is true. By singing a territory is defined and defended. Important business, as only a territory of sufficient size will allow survival of the owner and his offspring. That's also why no biologist would advise to shoot a male from an adjacent territory, as the claims are already stacked out and a working system is in place.
Point two can be answered by looking at it this way: By playing a tape recording we add another competitor into the picture. One that can not be beat, since it sings louder (thanks to amp and speakers) and more enduring (as long as the battery lasts) than the real thing. There is a fine line and if we overdo it the overpowered bird will eventually do what this whole mechanisms of territoriality is all about, it will give way to the superbird with "proven" superiour biological fitness. Problem is that superbirdy will not breed and maintain the species.
Tape recordings can be put to good use (bird counts, surveys, birdphotography) and do no harm if used knowledgable and wisely. Used without some knowledge about bird behaviour and biology a lot of harm can be done (during bird counts, surveys, birdphotography). Birds may leave territory, abandon nests and nestlings, neglect care for their offspring and themselves.
So if one needs to ask what songs to play, when to play them, and where, they shouldn't be doing it as there is obviously a lack of the know how to make it a save business (people doing bird counts and surveys mostly have this kind of knowledge, and so do a lot of birdphotographers).
However, for me there is a critical difference between a photographer taking pictures of birds, and a birdphotographer.
Ulli
