Except it isn't necessary and those Shepards would be wrong as proen by the epilogue. It doesn't matter what your Shepard thinks, your still killing your allies, petty justifications do not change this.
Epilogue? That's a meta reason. Your Shepard doesn't know what happens in the epilogues at the moment he has to make that choice. Think about the first time you were making that choice. You didn't know what was going to happen, same goes for your Shepard, every single time.
Besides, epilogue or not, you don't know what will happen with Shepard "controlling" the Reapers. There's no guarantee that, being the ascended god-like being he now is, he won't eventually reach the same flawed conclusion as the so-called catalyst. Also, what's to stop the Reapers from re-writing the digital Shepard once he gives up his life? And like I said before, attempts to control anything in the MEU (let alone something as powerful as the Reapers) have consistently proved to turn out absolutely disastrous. And if nothing else, control allows the Reapers to exist. That alone is an unjustifiable risk.
In Legion's words: "There's a non-zero probability of error."
When all sentient life in the galaxy hinges on this one decision, it's easily justifiable to say the risk is too big to not destroy them.
I don't understand why people think Anderson is the spoke person for paragon ideals despite the fact he is never shown to be. In fact, he is often shown to be quite ruthless.
Remember back in ME1 when Anderson explains what happened between him and Saren? You know, that mission where Saren ruthlessly killed tons of people and Anderson disapproves of it strongly? That brilliantly demonstrates how Anderson is exactly the opposite of ruthless. Remind me where he's shown to be ruthless again?
The we implies that at least the people you are sacrificing actually know they are making a sacrifice instead of being blindsided. Do you really think the Geth would sacrifice themselves in such a way when their are other options that spare them?
I actually think they would. In fact, if I'm not mistaken, Legion implies as much. EDI even explicitly says so.
The way I see it, synthesis will change (and thus kill) everyone (they will no longer be what they were). Aside from that, it does something that Shepard morally shouldn't be able to decide for everyone else. Control is too big of a risk. When seen this way, there's a good chance everyone will die in every scenario except destroy. When all sentient life in the galaxy hangs in the balance, and there's only one option that will actually save anyone, like a friend of mine put it: you don't have a choice to sacrifice EDI and the Geth, it is your duty to.