Now that Minchas Shlomo has explained to us that making a bracha is fulfilling a Rabinic obligation and not removing an issur, one could ask, "So what's the big deal about a bracha l'vatala?" In fact, a big "one" did ask that question, the Chayei Adam. The Chayei Adam notes that one could (and, in fact, it is praiseworthy to do so) say, "Atah, HaShem Elokeinu, Melech HaOlam, borei pri ha'eitz!"/You, HaShem, Our G-d, King of the Universe, creates tree fruits! In fact, you can say that all day, with all the variations: borei pri ha'gafen, borei pri ha'adama, sh'hakol n'hi'yeh bidvaro... How can it be, asks the Chayei Adam, that just putting "Baruch" in front changes it from a praiseworthy act of midas chasidus to an issur d'oraisa of taking HaShem's name in vain?!
Hmm... when you put it that way, that really is a very good question! Minchas Shlomo (Vol 1 18:10) has a very good answer.
There are four places in shmone esrei where one is required to bow; beginning and end of the first bracha, beginning and end of modim. Suppose someone wants to bow more? He really, really wants to show his deep devotion to and appreciation of the Creator. He's not allowed to; he needs to be stopped. In fact, even on the Yamim Nora'im he is not allowed to bow more. The Shulchan Aruch notes that in these days of awe, if one is so inspired that he really wants to bow more, then he just has to be sure to be standing erect at the beginning of and end of the bracha. Even more, suppose one is still davening his sh'moneh esrei during chazares ha'shatz; he should bow with the congregation when they do at modim -- unless he is at the beginning or end of a bracha -- in which case he is not allowed to bow with the congregation. Why? So it doesn't look like he is adding on his own bowings.
Tosefos explains that the problem is that others will see him and figure (seeing as he is so devout and pious) that all bowing is just for the devout and pious. His personal attempt at extra devotion will lead to community wide degradation of avodas HaShem! You don't want that on your plate.
That's the same problem with saying extra brachos... others will come to think it is a matter of personal choice and only for those who are moved to bless need to do so. That is a chillul HaShem of the highest magnitude!
One can use this principle to understand all the places where Chazal instituted rules for when and by whom "d'varim sh'b'k'dusha" should be said. Take zimun, for example. Nothing in the individual words themselves that indicates why even two people eating together shouldn't say the zimun. Or two men and a woman. Or saying kaddish with less than 10 men. Or a woman saying kaddish.
In all cases, we don't say, "What's wrong with saying it?" Instead we ask, "Is it appropriate, ie, according to the way Chazal established the practice, to say it?" If the answer is "yes", then it must be said. If the answer is "no", then by no means should it be said, and thus should people be instructed.