Yeah, the "problem" is that the built-in fonts are heavily space-optimized. The tracker has knowledge about the width of every letter in those fonts so the font tiles can overlap without occluding parts of the letters. With generic fonts, this is of course not possible, since they could have any shape. However, this feature was mostly implemented with High-DPI devices in mind, i.e. devices where you need a bigger font to read the patterns properly, in which case the bit of extra space is most likely not all that important anymore. And of course the people who always complained that OpenMPT's font is too small for them to read.