Grindclock is a hindrance because first of all, it is 3 mana and on a turn that you'd be playing a
grindclock and putting a counter on it, you could instead be milling your opponent with a
tome scour and
mind sculpt. So that's milling for 12 cards in one turn verses milling one card in two turns. Yes, multiple grindclocks out would be sufficient, but in the majority of games you play, you will not get out more than one.
Grindclock is really just another one of those artifacts that is more of a filler rather than a card that is constantly used in decks. If you want suggestions for cards to put in instead, here are some great ones I use in my bb mill:
Thought Scour,
Jace Beleren,
Jace, Memory Adept. Those are the cheaper ones, but if you have money and are willing to be risky, try
Mesmeric Orb or
Visions of Beyond.