Applying to PhD programs in mathematics: would a letter of recommendation from my department chair at the community college be of any advantage?
I agree, the letter from your current department is likely to be more meaningful than a letter from someone who barely remembers you. The people reading your file will immediately recognize you as a "non-traditional" applicant (i.e., not straight out of undergrad), and will not be surprised that the letters in your file do not follow the pattern of traditional applicants. 

While many employer letters are utterly dispensable in graduate admissions, it's a different story when the employer is a math department, "even" at a CC. Chances are that some of your target departments (at least those that combine pure and applied math) are struggling to staff their remedial courses. Slightly cynically speaking, they will be happy to have an instructor with faculty-level experience and TA-level salary (in addition to research prospects).