To speak plainly, you need to remove the pebbles from your mouth.
is ambiguous. It may mean
[In order] to speak plainly, you need to remove the pebbles from your mouth.
or it may mean
[If I may be allowed] to speak plainly, you need to remove the pebbles from your mouth.
In such situations, you do your readers or hearers a service by including the words necessary to make your intentions clear, as with
In order to speak plainly, you need to remove the pebbles from your mouth.
But setting aside such unusual situations, I also think that "in order to" is justifiable in many instances as a matter of stylistic preference, whether it is necessary to avoid ambiguity or not. Consider this instance from Henry Neill, "Thoughts on Atonement, with Remarks on the Views of S. T. Coleridge" in The Biblical Repository and Classical Review (July 1849):
Yet as the seed must yield up its old form and die in order to live, so a human soul must [y]ield itself up to God, as God urges His way into it, and thus cause old things to pass away, in order that all things may become new. The soul must, in fact, die in order to live.
You could remove the three instances of "in order"—two before to and one before that—and have no serious difficulty understanding the sense of the passage. But including the extra words does nothing to diminish the tone of the remarks.
More typical (and recent) is this example, from Alasdair MacIntyre (1998), quoted as an epigraph in The RoutledgeFalmer Reader in Philosophy of Education (2005):
It is one of the marks of a community of enquiry and learning that, while it cannot but begin from the standpoint of its own cultural and social traditions, what it is able to learn, in order to sustain itself, includes knowing how to identify its own incoherences and errors and how then to draw upon the resources of other alien and rival traditions in order to correct these.
Again the instances of "in order" are not strictly necessary for sense, but they are (in my opinion) thoroughly defensible as a matter of personal style.
That's not to deny that some writers make very bad choices in their use of "in order to." This excerpt from Per-Anders Tengland, Mental Health: A Philosophical Analysis (2001) is a kind of poster child for stupefying overuse of the phrase:
What I need to show is that these abilities are self-evidently needed in order to reach the goals in question (given the environment). Take the following example. In order to open a bottle of wine a person needs a certain agility in order to turn the corkscrew. She also needs some strength in her arms in order to pull up the cork. We might then say that it is necessary to have some agility and strength in order to reach the goal of opening a bottle of wine.
But this example merely shows that as stylists some writers have wooden ears. In my view, systematically eliminating "in order" from the set phrase "in order to" on grounds that it adds nothing useful to a piece of writing is unduly presumptuous and prescriptive—and I say this as a copy editor who has no love for literary lard. A writer may reasonably believe that using "in order to" improves the tone and flow of a sentence, even when the sense of the sentence is readily inferable from "to" alone.