ME3's three ending choices may have been a bit naff, but at least we all chose different things. Would anyone, faced with the dark energy dilemma, choose to side with the reapers? One is hope, the other is giving up.
I like dark energy as a motivation for the Reapers (I see why they changed it though - they wanted something that fitted the themes of the games more, it just didn't make sense) but I'm still not sure the Reapers needed explaining in the first place. Hell, the ending as is works fine/better if you just carve that bit of information out. Given that you can't argue or discuss it with the catalyst anyway.