I think saying none of the decisions mattered is a pretty silly honestly when its impossible to get the best case scenario's for the Genophage cure and the Geth/Quarian war, essentially the first half of the game, without not only an import but one that has the right conditions.
I would even go so far to say that yes - most of the decisions mattered on some level. Seeing characters like Mordin and Wrex for example that I knew and getting to complete their character arcs provided totally different experiences than when I ran through with Padok and Wreave.
Now if you're saying it didn't create a wholly different game in terms of levels and such - then no but that was never in the cards.
The dilemma that BW always faced was what do they do with the content that somebody might not experience because they made a different decision. The choice is either to make the game much shorter so that they can include alt content or to make it so that everyone can experience the same basic level of content so as not to penalize gamers for playing ME a different way.
They really in truth did no less than what they did w/ ME2 in relation to all of that. And there most certainly were people who leveled the exact same complaint against the second game.
Now this being the last game that's admittedly more problematic when it happens but the only places honestly where I felt they messed up seriously in regards to that was the Rachni and the Collector base.
You might be dissapointed for example that Jack or Thane are not squadmates, and in some cases such as Miranda I'll certainly agree that they could have gone further in terms of what was there but that is just the reality of game design - there is no game out there that has had to function as both a stand alone and a sequel in which the characters and concepts available can be vastly different from player to player, but when the core content is I would say immensely better with their presence to the point where its clear it was designed with them in mind, the idea that the game was concieved in mind for newbies doesn't gel to me.
Its definitely a thin line to walk and sometimes I think they fell on the wrong side of that line, but for the most part they did a pretty good job.