In various points in the game there are "log entries" similar to terminals except that there is no onscreen indication of any progress or benefit by accessing them. I realize that they provide some "flavor" with background context of the plot - but is there any benefit to collecting them all, beyond that?
Short and simple. These data pads don't have any known benefit for your effort. They add context and extra bits of story.
Like I said, there is no known benefit for it. But there are plenty of hidden items in Halo 4 to keep us occupied, so that isn't to say there may be some benefit we don't know of yet. Like the finding of the working pelican, or even the RvB easter eggs, there may be more than meets the eye with them.
I haven't played Halo 4 yet, but I know Halo 3 had terminals as well that gave you some info. I assume it's for the same reason of an achievement for getting all of them and also for some background info. Edit: I've now actually gotten the game and there is an achievement for getting all terminals and an achievement for getting your first one. Also it says how many terminals you've found in the campaign section of your waypoint stats. it's just for the achievements and bragging rights. Also there are two types of terminals, the kind that tell you master chief's history, which are for story purposes only, and the other kind, which are harder to find are the achievemet and waypoint ones.