The only thing about that is it would have to drip all day long unless you manually turn a valve every day. You don't want to be dosing ALK and calcium at the exact same time either. Finally, how do you adjust it so that you know exactly how much of your additives you are putting in? Kind of hard to figure that one out...
With dosing pumps, you know the exact dose per minute. Plus, you can have them dose at different times to avoid putting multiple additives in your water at the same time. Also, everything is fully automated. All you have to worry about is re-filling the additive jugs...
I use the BRS dosing pumps and they work great! I run them off some cheap Digital timers from Canadian Tire. Just set and forget. Adjust every month or 2 depending on your coral growth.... A couple hundred bucks to do it this way but totally worth it. Especially, when you think of how easy it is to crash your system with high ALK, etc.
