Gentoo is not exactly a systemd-less distribution—you do have the option of installing it instead of going with the default (OpenRC). Personally, though, my Linux use predates systemd and no one has ever presented what I would consider a compelling reason for me to switch to it, so I install all my systems with OpenRC. I've reached a stage in my life where I dislike unnecessary churn in utility software.

From a less-personal standpoint, I consider the philosophy behind systemd to be wrongheaded (as in, it looks very much like Microsoft's embrace-extend-extinguish—and no, I do not trust Red Hat not to be sold to someone who will take advantage). Many of the other reasons-not-to-use given by others here also ring true to me.