I mentioned the issue with sandwiches in a thread not too long ago in the card magic forum.
As far as the spectators go - visualartist is correct, they notice more than you think. And while they probably will never think "Why is he using two cards to find one?", I believe that discrepancies, inconsistencies and little things that don't make sense have a tendency to subconsciously place the spectator into a heightened state of awareness, per se. I don't want to get too psycho-analytical here but suffice to say that an effect which is unjustified can destroy an entire routine. For one, it will interrupt your continuity as a performer and hence detriment your credibility. It creates doubt and cynicism towards your patter, and towards the general mood of your routines. This in turn hinders rapport which is essential for the full success of magic.
Unjustified routines make it that much harder for you to do your job. Even if they never consciously think about it. Think about it - if you had a mentalist performing to you, explaining that he was reading your body signals, and then started performing metal bending - well it's probably fairly obvious he wasn't reading your body signals before, was he? He certainly isn't reading the spoon's body signals. The performer and the performance rather than the effect specifically suffer.
And one final thing - ok, so you're showing me something cool you can do. So? Who cares? It will still be cool, but magic can be much more than that, and I believe that the majority of "Just because" effects don't reach their potential because of it. Just because is great - it's awesome. But magic's effect comes from the spectator's involvement; just showing them something you can do just because you can do it will never be enough as justification.