A few things:
Considering that the reapers were arriving anyway, it's a safer assumption that the purpose of the human reaper was simply to be, well, the human reaper. The Collectors were going to harvest enough people to build it, then the rest can simply be destroyed once the reapers finally arrive. Of course, this begs the question as to why the Collectors have to do this at all, since the reaper fleet is well capable of doing this themselves. The Citadel relay plan failed. It doesn't seem sensible to bother making this same desperate move again, risking the destruction of yet another reaper, particularly one fresh off the macabre showroom floor.
Creating a human reaper to open the Citadel relay is just one possible scenario, one that I thought might be likely. Who knows, maybe the Reapers thought a new (sneak?) attack on the citadel might be successful with the Collector's help? They were certainly a bit desperate to finally re-enter the galaxy as soon as possible, at least that is the feeling I had in ME1 + ME2.
The human Reaper could have also served another purpose, hard to say what it might be though, because it was not clear what the Reapers were planning to do with it. They left it up to the player to figure out what they might want with the human Reaper, the only thing that was clear is that a (new) Reaper is a dangerous weapon, which needs to be stopped, no matter what they might be planning to do with it.
Shepard's death and subsequent resurrection is pointless and stupid, because it's perfectly interchangeable with a 2 year long coma within the narrative. Hell, even the characters themselves seem to be pretty inconsistent in the way they talk about it. Shepard says "I died" then it's "I almost died" and Zaeed talks about his/her surviving the Normandy's destruction. The time away being nursed back to health in some secret facility while the leaders of the galaxy sit with their thumbs up their butt was sufficient to force the temporary Cerberus alignment. Maybe it was all an elaborate ploy to make a geth unit unique by sticking on some broken N7 armor. Killing a character off, then bringing them back should provide something unique to the story, but it doesn't. It's simply a Shepard and Normandy reset button.
Nobody other than Shepard recognized the Reaper threat. The leaders of the galaxy thought that Saren Arterius was the sole menace and therefore believed that his defeat freed the galaxy of any peril.
Don't forget that Shepard was indeed special (Cerberus knew this, that is why they resurrected him). Shepard touched that Prothean artifact at the beginning of Mass Effect 1, which in turn gave him the ability to recognize the Reaper threat.
I thought this story element was fine and really enjoyed the situation it put Shepard and the galaxy in. Anyway what is wrong with Shepard dying and then being resurrected? I don't think there is anything condemnable about it. They could have done something completely different, but why? It was pretty cool and also showed the player how powerful and technologically advanced Cerberus is.
Anyway, if anybody means that the ressurrection in itself is ridiculous:
Shepard's body was preserved on the icy planet (probably in methane or something, not water). In theory it might someday be possible to resurrect people who have been cryogenically frozen. Why not in a few hundred years when technology has drastically advanced?
The idea is certainly a bit far fetched, but so is space travel at light speed. Nobody can say if either technology might be possible someday or not, so suspend your disbelief and enjoy the SciFi fiction.
While ME1 does establish that the reapers are there to "impose order on the chaos of organic evolution", that seems rather incomplete. Why should they care? What's the point of imposing order? Are they simply galactic neat freaks that get off on making sapient life operate like clockwork? In all honesty, I wouldn't have cared one iota if they never answered these questions, because the ultimate goal behind their will to impose order was never really all that intriguing to me anyway. I would have been happy enough to just kill them without ever really knowing everything behind the curtain, because it satisfies the portion of my brain that's part lizard, part caveman.
We did not know the Reapers true motives until Mass Effect 3. The Reapers were so mysterious, I loved it and enjoyed this story element immensly... and then Mass Effect 3 happened. *facepalm*
It would have been much more satisfying to just completely wipe those suckers out without ever knowing their true motives. I dunno, maybe they maybe they are ancient robots created by an ancient race, that just went completely psycho for no apparent reason, kind of like HAL 9000 in
2001: A Space Odyssey (okay HAL 900 is just a computer)
or Skynet in
The Terminator. That would have made a better story than the BS that Mac Walters created in Mass Effect 3.