Why have such a bloody Suicide Mission?
Well, to start with, it's a suicide mission. While I know many who disagree, I am a firm believer that the only thing less plausible than overcoming impossible odds is overcoming impossible odds without significant costs. And it's also one of the themes of Renegade Reinterpretations: victory comes with sacrifice. People, ideals, whatever. Shepard lost a team member to Saren on Virmire, and the Collector Base was supposed to be far worse than that. Now it is.
One aspect about character death that undermines the effects of 'optional' deaths is that people feel like any avoidable character death is a punishment. Often it is: less story, less satisfying substitues, or simple lack of content. A good example is Wrex and Wreave: in ME2, Wrex's substitute is not only worse for the galaxy and the Krogan, but he's also worse for the player: none of the Wrex-awesome. It's definitely the 'worse' alternative. But when a character death is absolutely unavoidable no matter what, it loses much of its weight due to foreshadowing and unavoidability: it's far harder to hold weight, and can become the cheap sympathy gimmick. Who felt bad about the death of Nihlus?
But when deach is affectable, but not avoidable, you get into some pretty potent territory. The reason that Virmire stands out in ME1 is because of the Ash/Kaiden decision, not Wrex. Wrex's death is avoidable, easily so: have the persuasion check, or do his quest. Once you know about it, it's easy to avoid, and so choosing the 'bad' alternative and killing Wrex has to be a deliberate choice. The same applies with deaths in the Suicide Mission of ME2. But with Ashley and Kaiden, it isn't avoidable: no matter how well you fight, how high your paragon/renegade bar, how awesome your upgrades, someone will die. But the player is still involved: the one who dies is up to you and your influences. Neither Ash or Kaiden is guaranteed to die, but one of them has to.
Of course, this isn't hard for everyone. Some people hate one or the other. Understandable, and unavoidable. If you like one but not the other, it's hardly tough. But with the suicide mission, you have a larger group of people, and a larger group of potential sacrifices: while it may be easy to not care about any one of them, few people will want to see a third of the characters bite the dust. Fewer still will be able to easily arrange which third will: imagine the people who try and leave two unloyal/wounded Vanguards to cover the retreat because they want them to die, only to lose all their survivors as well.
Hands up: who romanced a character their first time, and then stuck them as a mission specialist because you thought they were awesome and you wanted them to prove it? Wrestled with Garrus, and then put him as team leader both times? Did snu-snu with Tali, and then trusted her to do the vents and get you through safely?
Now, who would be affected by hearing Tali burn alive in the vents so soon after a romance scene? Or trusting Garrus to keep it up as a fire team leader twice, only to see him fail? Or someone who romanced Thane and sent him as the Assassin, believing that somehow he'd survive certain death?
There's an element of unpredictability, unexpected results, and far more variation to make the mission feel dangerous. The Suicide Mission is no longer 'did you have everyone's loyalty and pick obvious best choices for a perfect ending': when perfection is impossible, and there is no shame in losing people, the question is preference and results.
Ask not 'did you lose anyone': instead ask 'who did you lose.' The first is a question of success and failure: the second has no stigma.
