There are much easier ways of getting said basic components than going around mushing up living creatures. Possibly it's a throrough but destructive scan (although it doesn't look remotely like that, more like crudely destructive). I'm sure they probably do preserve species in the form of a series of records of that species, perhaps in enough detail to (in theory if not in practice) recreate it and its culture pretty accurately.
According to the lore the genetic material is in itself what stores all the knowledge, memory and experiences of the people. In Mass Effect your memories, knowledge, experience (essentially everything that makes you who you are) are all genetic markers that can be read and stored to hardware. The very 'essence' of the species is saved. Stuff like this goes back as far as the first game.
I agree, there are much more efficient ways of getting material to build reapers. Along with the fact I'm sure if you wanted to store the genetic information of races then there must be easier and more efficient ways of doing it rather than creating a reaper.
Do you actually know what reapers are? Also tell me more about your easy ways to store all the memories, knowledge and experiences of entire species. I wasn't aware that this possible outside of fiction land but clearly there are easy and efficient ways to do it. Fascinating.
You're not doing the Reaper's justice if all you think is that they do is store the information. Its more complicated than that. The harvested cycles ARE the reapers. Each reaper is a nation, a billion organic minds all linked together to form one entity. Similar to how the geth consensus works, imo.
If you're actually interested in this Mass Effect lore may I recommend you read the Reaper wiki page? It will only take 10 minutes of your time.
http://masseffect.wi...com/wiki/Reaper