Personally, I find it interesting to compare this question to the analogous one with respect to pickstyle: i.e., downstrokes-only vs. strict alternation.
When I started out I focused entirely on fingerstyle using the traditional (alternating) two-finger method, so that became natural for me and I rarely deviate from it. When I decided many years later to learn pickstyle, the analogous technique of alternate picking came naturally to me for more-or-less the same reasons, in particular efficiency.
But when thinking about it I also discovered an important difference. One reason I enjoy playing with a pick is the physical up-and-down motion in the wrist -- like a rhythm guitarist, so to speak -- and as much as possible I like to play downstrokes on downbeats and upstrokes on upbeats. When playing straight fours or straight eights, etc. this is the same as strict alternation, but when the line doesn't follow that rhythmic pattern I tend to go with the downstrokes-on-downbeats rather than strict alternation. I suppose technically this would be like using your index finger on downbeats and middle finger on upbeats (or vice-versa), but to me it doesn't feel the same. When I tried to play fingerstyle using the index-for-downbeats method it just didn't work for me in the same way as the downstrokes-on-downbeats method does with a pick, so I still tend to strictly alternate with my fingers independent of the upbeat/downbeat distinction.
Does that make sense?