For me, I was quite happy using syringes and pipettes for years and didn't even think about mixing by weight, but as vaping became more popular, and people I know wanted me to mix them juice, I felt that my juices had to be exactly the same every time.
A squirt and a guess wasn't going to cut it really if people were giving me their money.
But once I started using scales I also found it to be far easier and quicker and there is less washing up, whilst having to do a bit of washing up isn't the end of the world its also nice not to have to do it at all.
I mix to 0.01 of a gram, Im almost anal about it, yes I know that .01 of a gram really wont make any difference in fact I would say that apart from maybe a couple of flavours 0.1 of a gram wont make much difference, but it is 1% in a 10ml mix and sometimes that can be a lot.
Anyway, do what makes you happy is my view.