Helvetica and Times both significant glyph variations between the normal and bold weights; the former in particular glyphs most notably 'a', the latter in the entire oldstyle vs modern shape category. This kind of thing wreaks havoc with simple linear interpolation. It will be interesting to see how, ahem, Macromedia (right?) deals with this.

Another possible interpretation (cue spooky music) is that putting renewed emphasis (npi) on MM fonts is a symbol of independence from Microsoft. My understanding is that MM is effectively an optional part of the OT spec, and that MS's renderer doesn't deal with it.

MM technology is not an optional of the OT spec. It was removed from the spec over the objections of Adobe and most designers.

MS's renderer doesn't deal with MMs? Just drop one into your Windows font folder and see the instances appear in your menu.

I agree that it is unfruitful to MM-ify either Times or Helvetica unless they are substantially redesigned. Nevertheless, we will see someone's interpretation in a surprising place quite soon.