Dorian is not "C major starting on D"; it is a tonally-rooted D scale with a different set of progressive intervals; it may correspond to a shifted C major scale but decidedly is not major (either in scale or in the third). This is especially apparent if you attempt to harmonize Dorian tunes: the tonic and dominant are naturally minor chords (though sometimes major chords can be used, at chromatic variance with the natural scale). Because the underlying harmonic structure and progressions are different, it deserves study as a distinct entity. The different intervals even mean that the melodic forces are different; for instance, the 7th, being a whole step from the tonic, has less pull toward resolving there.

Ignoring that "modal" historically refers to just temperament, the frequent chromatic variances in harmony are one reason Jack tends to cringe when people talk about "modes" rather than "scales" in much folk music. A perfect example is when people call minor tunes "Aeolian", even though they might use three different scales when performing them: natural minor, melodic minor and harmonic minor (the last of which may refer to separate scale patterns ascending and descending!).

There are also other modes/scales which do not conform to the major scale, such as those found in blues and in gypsy music, where a scale might have both an augmented fourth and a minor seventh. Quite a number of folk tunes have indeterminate modes, either because they fluctuate between modes (say, Dorian and Mixolydian) or because the scales are gapped. And you may find the same tune played Dorian in one region but Aeolian/minor in another--starting with the same tonic note (and therefore with different major key correlates).

Another reason to study modes is so you don't confuse other people when you talk about modal music yourself. I've seen quite a few instructional books where the author calls a tune C Dorian when the tune is actually in D Dorian--and gripes when others "cause confusion" by using the proper name! And way too many people notate modal music as if it were just major or minor with accidentals, rather than using the proper key signatures to convey the true nature of the music.