Really!? Oh my god, I'd been grabbing the entire franchise from the wrong end of the stick for ages! Thank heavens you've come along and set me right.
That was actually aimed more specifically at this;
Once a character's recruitment is out of the way, and their loyalty mission if you choose to do it, their part in the game is gone and they might aswell be replaced with a faceless goon because they won't influence the story in the slightest.
-----
Except for the fact that you do interact with them multiple times on their loyalty missions, this impacts the story just as profoundly as the loyalty missions for anyone else. The ending conversation between Shepard and Kasumi, for example, is one of the most personal and moving exchanges in the entire game, an example of character interaction and development that far surpasses Grunt's mindlessly violent quest for identity or Miranda's generic 'defrosting ice queen' development.
The story does mean Shepard vs Reapers, I'm sorry to say. Everything else is completely and utterly optional. You don't HAVE to recruit Thane, Morinth/Samara, Tali, Legion, Grunt at all and some people won't. But that doesn't change anything. The only thing that changes as far as the fight against the reapers, is that the respective doors to their rooms on the Normandy will not open. And yeah you can make friends or allies depending on your own preferences. But unless you are going to romance somebody, that counts for absolute shit. If you're not romancing Garrus, Tali, Miranda, Jacob, Thane, Jack; then they will say the same lines to you EVERY single time you have a playthrough of this game. That right there is no different to Kasumi or Zaeed. You can't have varying levels of connection with them in most circumstances, you just choose how many times you are going to talk to them. Their level of connection with you in this sense, changes absolutely nothing.
Most of your interaction with Kasumi on her loyalty mission consists of her telling you what to do next to bust open the vault, or commenting on the treasures in Hocks vault. The only meaningful conversation you have with Kasumi is the bit at the end of her loyalty mission.
With Zaeed you get to choose whether to botch his mission or not aside from those two things, and the fact that both can die on the suicide mission, they have no other reason for being in the game, while almost everyone else has a recruitment/loyalty mission with about that same level of interaction. Not forgetting of course that there's a whole new conversation tree aboard the Normandy after every second mission (usually) which gives you a far greater chance to get to know more about each character, make them like you more or less, pursue a romantic relationship, upgrade the Normandy's weapon, shields or other functions. They all have a lot more deeper involvement in the game, and in Sheps life, than Z&K do.
With Tali you'll get to learn more about life on the flotilla and her own childhood, she'll talk more about the affect of her loyalty mission and what she thinks of her father, she talks about ME1 a little bit.
Samara explains more about her daughters and her life before becoming a Justicar and small nuggets about life in Asari space.
Mordin for christ sake, starts off talking about all this scientific gobble-dee-gook and over the course of the game, and many different conversation trees he warms up to you enough to sing a song from the Salarian version of Death of a Salesman (or something.)
The point is, all these characters grow and give you so much more insight into who they are and how they view life compared to Zaeed and Kasumi, and all this takes place on the Normandy over a natural (for a game) progression of time and regular interaction, not all blurted out at the end of a mission in a five minute cutscene.
Like you said man, characters loyalties effect who lives and who dies at the end of the game, and these two characters are no different. Hell, its entirely possible to end up killing off Zaeed entirely, which can drastically reduce the options available to you in the suicide mission.
Yeah they can die, but without having anywhere near as much of a chance to know anything about them other than 'Arsehole nicked my job' and 'bf died
' (I know that's an oversimplification, but compared to other characters that is essentialy all they boil down to) even if you wanted to, their deaths seem to pale in comparison
To start from the bottom. Seducing Morinth DOESN'T affect the story. It has about as much to do with the canon or plot, that getting killed by a goomba on a level of Mario does. You get the option to reload the game. The credits don't roll, therefore it isn't the end of the game/Shepard's journey. You won't be able to carry that choice through to ME3 at all, so it has NO effect on the story whatsoever.
Also to go on the fact that you said character's loyalty affects who lives at the end and who dies? The same goes for Kasumi and Zaeed. But guess what, if they die at the end alongside any other squad member, how is this going to change the reaper invasion in ME3?
Oh dear, Grunt died in my game. I needed him to convince the Krogans in Mass Effect 3 to side with me and win the war. Now the ending to Mass Effect 3 has already been decided for me. No. Not going to happen.
Mostly because you'll have a much better chance convincing the Krogan to side with you by going through Wrex, who is respected (and their leader, that's a big plus), while they mostly just see Grunt as a lowly tank-bred ... thing. ;D
And who lives and who dies won't affect the Reaper invasion one little bit (probably) but it will affect the course of the story just as much as anything else, because if you pay attention, the characters all reference each other, they have their own friendships and rivalries with one another. If certain characters die and some others survive that's going to change things in ME3 probably in ways you wouldn't even think of. Small things mostly, for sure, but do you really think Bioware would make such a big deal out of the fact that everyone is expendable if it didn't have some significant effect on the next in the series?