I agree with that but the hole might be too big in some designs as thermal shots and videos show. Also hole size should be a function of coolant fluid viscosity and flow rate which in turn depends on your pump. I think there are too many variables involved to just drill a hole between reservoirs. That's why i think some way to adjust it should be included. It's intended to be used by enthusiasts after all. On the other hand i feel that flow resistance shouldn't be adjusted by means of a bypassing hole but changing fins design. For example they could connect all fins in parallel instead of two columns in series as that would halve flow resistance. Of course changing manufacturing processes IS expensive and using a hole is fast and cheap... Yeah, if your loop is performing well you don't have to worry about it but it could probably perform better. If you spent lots of money on it you will want to get the best out of it =3 .