ever see the documentary thousands in an hour? Them SWAT boys were under extremely heavy fire and they used shields. They're ballistic shield for a reason. The Shilds you're talking about are Riot shileds and they are not ballistic. We are referring to the ballistic kevlar lined carbon alloy jobbers with the view window.
Even going with your statement, "they are only used for frag and schrapnel," what about grenades? What about tripmines, door breaches, landmines, and the ever infamous Flamer debate? If a Flamer is in play, and a shild could protect you, would you not want one to keep from getting your marker covered as you said you'd be mad to have to clean your gun out...
Again, I'm not saying your points aren't valid, I'm just saying your not looking at all the angles. REDS use shields, and they're available for SEALS and we all know SWAT has them, and use them often, especially the ones here in Delaware. Remember, there are doors to be breached in this league...