Whether it is dynamic time warping or some sort of Euclidean k-means clustering of a time series, it is (nearly?) always required to consider irregular spacing of data, unequal lengths of data and/or missing-ness of data.
While realizing that each of these issues have considerations unto themselves, is there a general reason why pre-processing each time series with a spline to interpolate (or very minimally extrapolate) the data to ameliorate these issues cannot be done?