I like readdlm() and writedlm() because it is easy for starting with Julia and arays. And the functions are used in a lot of examples, packages.
For decimal comma I wrote a packages (“ReadWriteDlm2”) which also use this functionality and sytax: https://github.com/strickek/ReadWriteDlm2.jl
I personally find the idea of having readdlm and writedlm in Base working nicely with Matrix{<:Number} great. I don’t want to install a bunch of dependencies for saving the most simple built-in scientific type in a language. Arrays of numbers are everywhere, we just need a formatting option in these *dlm functions and everyone will be happy.
If on the other hand, someone needs more fancy control other than formatting like Nullable handling, etc, then a package makes sense.