Should I ask a letter of recommendation from a prof who I rarely see?
A question you have to consider is - does the Professor know enough about your work, work ethic, attitude etc to be able to write a strong and meaningful reference for you.  A 'vague' and 'form-letter' type of reference could potentially harm your application, if it is the main reference you use.

Having said that, it would not hurt to ask and obtain a reference (if the Professor is comfortable doing so) to add to your portfolio.