One issue that may be relevant is that "could" also implies possibility or uncertainty in some cases. If you ask me if I will edit a spreadsheet for you and I respond that I will, then there is no uncertainty. If I respond that I could, there is an implication of uncertainty. If I were to include both "might" and "could", I am using two words that both suggest uncertainty.

With regard to a previous comment, it is inaccurate to suggest that individuals who use a term when speaking will not use that same term in their writing. While it may be less common to use some informal or regional terms in writing, I have seen many students and co-workers use informal terms in their writing.