Just my two cents...
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I get the feeling that the Reapers are collecting humans for a much greater purpose than reproduction. As already mentioned, they're immmortal, and the inclusion of organic material in their makeup could probably be bypassed by such a technologically-evolved race. I was always confused as to why Reapers were half-organic and not fully synthetic... why the hell do they need to incorporate organic components at all?
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Harbinger's line was: "We are your salvation through destruction."
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Does that mean that the Reapers might actually believe that they're ensuring the survival of various species by storing "backup" genetic material? The massive amount of matter contained within each Reaper (millions of humans were to be harvested) is easily enough to maintain and/or reconstruct numerous viable genepools of the species from which it was created, and the Reapers' survivability and sheer power would ensure that the precious organic cargo within each of them would be protected from external harm, even against forces originating from beyond the galactic border. Maybe they're nothing but gigantic, armoured, armed, self-aware gene-storage facilities for some unknown, even higher power...
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Or a second possibility; maybe they're a race of 'hunters'? Maybe, each 50 000 years, they emerge to kill and recover genetic material from all intelligent life in the galaxy? This, to me, seems plausible; modern-day hunters and poachers stalk animals through the wild; when they make a significant kill (such as a bear, lion or moose), they hang the animal's severed head up on thir wall as a trophy. Maybe this is the Reaper equivelent. Once they have their trophy (a construct in the image of the intelligent species used to create it, and containing a massive sample of their collective genetic material) they begin to eradicate all traces of intelligent life. This ensures that when they wake after 50 millenia for their next hunt, the galaxy will be populated by completely new intelligent life forms to stalk, all easy prey for the God-like Reaper fleet.
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Maybe it's even a kind of test; they could be searching for a new species that is adaptable and strong enough to fight back and reist their hunt. Perhaps they are searching for the "perfect organic race", or an "evolutional prodigy"... after all, Sovereign itself stated that the mass relays had initially been created with the sole purpose of forcing new civilisations down the paths the Reapers had intended... who's to say what they were trying to achieve by doing that? It could be an experiment on a galactic scale; when a scientist returns to his project only to find it a complete failure, he/she disposes of it and begins again. According to Sovereign, intelligent life "was an accident". Maybe 'life' is just the Reaper equivelent of 'penicillin', one of the most important medical developments in recent history (and also discovered by accident). Maybe the Reapers CREATED life, and are researching its origins or development, pruning it back to the roots every 50 000 years so that they can repeat their galactic experiment...
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Or maybe they're just cold-hearted, psychopathic, genocidal, mechanical bastards who want to kill everything with a pulse out of sheer spite. Don't forget that possibility.
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As for Tali, I think of her as a sister character, someone for Shepard to protect and help out, and rely upon when in need of support. Just throwing it out there... you know...