If a grub tapers too much near its head, then it will split wide open when you try to slip it over a jig collar. And if it does not split open, the lack of plastic in the head does not provide tension to keep the grub securley in place. It will slip down off your jig collar every time you cast or swing 'n miss on a hookset. Droopy drawer grubs are a real hassle for you, and a real turn-off for the fish in my opinion. Also, a fish will tear up the thin head section of a pin head grub in no time, and it's a poor choice for a Texas Rig in heavy cover.
Heavy cover is nasty and grabby.
Heavy cover will literally tear the hook out of a pin headed grub in no time. So I basically avoid pin head grubs for jigs and Texas rigs. The thing I really don't like is when the the body of the grub just tapers right into the tail of the grub so that there is no clearly defined hookin' spot on some brands. Aside from the superior bait designs developed by Gary Yamamoto himself, all Yamamoto grubs have an advantage over many other vendors' grubs in that Yamamoto baits are heavily salt-impregnated using a patented process. As an alternative, other manufacturers may coat the outside of their baits with salt, but this is an ineffective process that quickly dissolves the surface salt off of other vendors' baits in water, and does not have any lasting value as far as the surface salt being a fish attractant.