Today my counterpoint class got waylaid by a discussion of movable-Do solfège versus fixed-Do. One of my students has absolute pitch, and is completely convinced that fixed-Do is the way to go. She also studied in France for a year, so she may be influenced by the Conservatoire system. As I've written before, fixed-Do solfège relies upon a strong pitch memory, and this memory requires constant reinforcement. My student tried to argue that fixed-Do is an interval system, but it is only a generic interval system. The C - E major third is sung "Do Mi," as is the C# - E minor third and the C# - Eb diminished third. So at best, singing "Do Mi" signifies a third, but cannot specify which kind of third it is.
I teach my students that solfège is meant to remind one of aural images. Movable-Do solfège reminds the singer of tonal functions. Tonic is Do - Mi (or Me) - Sol, Dominant is Sol - Ti - Re. This holds constant regardless of the key, so students can rely upon the aural image of these functions to aid in singing. Fixed-Do solfège reminds the singer of pitch classes. Tonic functions switch with each new key, but pitch names remain constant.
I actually use both systems in my own teaching. My students learn Movable-Do syllables, and also sing on letter names without accidentals. The latter system is exactly like Fixed-Do in that a C# and a C are sung exactly the same way, but without translating from the English name to the Italian name. I emphasize the Movable-Do more than the letter names, but being aware of specific pitches while singing is of practical benefit to instrumentalists. The letter system also allows the practice of clef reading and transposition. I actually got this idea from Bill Marvin, current director of aural skills at Eastman. Bill uses movable numbers (One, Two, Three, Four, Five, Six, Sev) to indicate the scaled degrees and uses the Fixed-Do syllables, so switched from my practice. As Eastman has a large number of students from countries that use solfège syllables as note names, this makes some sense. However, it is rather difficult to sing numbers with any speed, given the ending consonants of the last four numbers. Letter names also have that problem (try singing "eff gee" six times very quickly), but as I emphasize the movable system more than the fixed system and the letter system has only one note that ends with a consonant, it still ends up winning the contest.
To me the most important thing is that my students can sightread well, can look at a score and play the music in their head, and can listen to music and recreate it on a staff or on their instrument. For the 95% of students who do not have absolute pitch, movable-Do will help in these tasks; fixed-Do will not.