it's only useful with custom templates, not modified I think

hmm, i don't think so, we don't have much custom templates, but a lot of modified ones, and it's very usefull

then when child styles' templates got modified and you have a lot of child styles you still need to edit them one by one. My child styles are unique one by one, I can hardly edit one parent for all.

It's only not very useful if every template in the child styles is unique.

If not, its still useful for any template that is not unique.

of course, if every childstyle is totally unique, then it wouldn't help.

It depends on the situation of course, but for example here on vb.org it is very usefull for normal templates