I think it would be just as well to list the variants of the base font as well as a flag indicating if their definition can be embedded with the document. Underline doesn't change the font outlines, so that's an easy thing to implement.
Arial has nine variations. Of the fonts I have installed, about fifty percent have no variations, including the Wingdings series. MS Office doesn't need to switch between the font variants. I'm pretty sure** it just sets flags, the base font and size and color and the OS creates a bitmap and some metrics about how big the bitmap is. There isn't anything the Office application does that depends on the variants, so it's not so complicated.
To support vector outputs, PTC needs to duplicate all the work that MS and Adobe already did on font rendering, but to convert the TTF definition language into PTC managed entities. PTC's done some of it, but to handle the cases where Bold or Italic are requested and not part of the font definition is going to be additional effort, especially since the result won't match what other companies produce.
**This is the Windows OS description of the Font and Text API at http://msdn.microsoft.com/en-us/library/dd144824%28v=vs.85%29.aspx From there: "When necessary, the system synthesizes a font by changing the character bitmaps. To synthesize a character in a bold font, the system draws the character twice: at the starting point, and again one pixel to the right of the starting point. To synthesize a character in an italic font, the system draws two rows of pixels at the bottom of the character cell, moves the starting point one pixel to the right, draws the next two rows, and continues until the character has been drawn. By shifting pixels, each character appears to be sheared to the right. The amount of shear is a function of the height of the character."