Because I would rather not "just buy a headstall" when all I need is a pair of cheek pieces, which, theoretically, should also cost a lot less than buying a headstall or an entire driving bridle! It's not that money is the issue, it's just that I already have so much horse stuff (show stuff, everyday stuff, training stuff, extra stuff, full/horse-size stuff, cob-size stuff, English stuff, western stuff, stuff, stuff, and more stuff!) and not enough space to store it all already! And, I already have a well made, perfectly useful, otherwise lovely, and already matching driving bridle; plus, all I can find is English or western (riding) style bridles, not open or blinder-less driving bridles. I might still want to be able to use a sidecheck (I am training a dirty little grass snatcher right now!) or an overcheck (doubt it, but never know) and a bridle really ought to be made to accommodate those devices (sidechecks usually feed through rings that are sewn into the crown of the bridle, overchecks usually feed through slots that are sewn into the crown, and sometimes into the browband, of the bridle). I found an English style bridle that I like (and that's probably the way that I am going to have to go), but it only comes with silvertone hardware, not brass or goldtone, and none of the western style bridles that I found came with cavessons nor do they match my cavesson nor do they match any other cavesson that I was able to find nor did any of them have brass or goldtone hardware.
And, I agree with you, I do not want to destroy a good driving bridle! I am going to want to use my harness (and possibly its matching bridle) on more than one horse and this harness (and its matching bridle) is nice enough to also show in (if I continue to take care of it). Aren't blinders required by some of the Miniature Horse breed/registry shows? I try to take good enough care of most of my everyday using items that I can also use some of them (where appropriate) at shows. I would love to just buy a training harness and maybe some day I will, but, right now, I just can't justify it with the few horses that I am planning on training/working. I would love to know more details about your Silver Fox headstall. Is it an English(riding) style bridle? Are you happy with it? Would you recommend it based on its quality (quality of leather and construction)?
Why not just buy a headstall? Can you not find one with brass hardware? I would not destroy a good driving bridle! Headstalls are fairly inexpensive. You may want to use the blinder bridle sometimes, so having two bridles is a good idea.
Maybe I am not understanding your question correctly.
Silver Fox makes a miniature leather headstall. I have one.