Commenting on the strangely short prayer by Moshe on behalf of Miriam, Rashi (second pshat) writes that Moshe was concerned lest the people say that he immersed himself in prayer for his sister but not for them. While the people were fast developing a track record of crying wolf and complaining without basis, this one surely is way beyond the pale. What are we to make of it?
I want to suggest a limud zechus and a different way to read Rashi. The meforshim are troubled by the extra word "leimor" in the pasuk, "VaYitzak Moshe el Hashem lei'mor..." "Leimor" suggests that the words are meant to be spoken to others, as in, "VaYidaber Hashem el Moshe leimor," yet here Moshe is speaking to Hashem, not instructing others. Why the extra word?
Perhaps there was a dual nature to Moshe's tefilah: on the one hand, Moshe was acting as Miriam's brother and offering a personal tefilah on her behalf. On the other hand, Moshe was also the shliach tzibur for klal yisrael, who, as evidenced by their willingness to delay and wait for her, were obviously concerned and moved by Miriam's plight. Klal yisrael wanted to join in tefillah as well. "Leimor" teaches that Moshe recited his tefilah out loud, encouraging others who wished to join him in a communal tefilah.
Most of us have probably experienced at one time or another a ba'al tefilah that went on a little bit too much and lost the tzibur's interest in the process. Moshe Rabeinu avoided a lengthy tefilah for precisely this reason. He did not want to be charged with having only his personal needs in mind and forgetting his role as the people's leader in prayer as well; he did not want people to say that the tefilos he offered were his alone and were not on their behalf as well.