Arguably the most vital part of a traditional roguelike's interface, a message log gives the player all those details that might otherwise be depicted by (or lost in the confusion of) greater graphical detail. It's also a nice after-the-fact record of why you find yourself flirting with the brink of death when things suddenly start to go south.
Message logs also present another dilemma unique to games attempting to "modernize" the roguelike tradition (for the first dilemma discussed earlier, see here).
Any game interface needs a primary area where the player is intended to focus the majority of their attention--all other areas are supplementary, otherwise it's tiring and difficult for the average player to constantly divide equal attention among multiple UI elements. This is especially important in games that are (or can be) played quickly, including both real-time games and roguelikes, lest the player miss important information (a leading cause of sudden death among RL players! ;).
While in most map-based games the map area is the focus of attention and any HUD information is secondary, one could argue that in traditional roguelikes maps are solely a way to tell the player what is where, something you need only glance at as a reference to understand the more important information contained in the message log: details of all the events that have transpired since the last turn. (Parts of the game where you're just moving around and nothing special is happening are irrelevant here since they're just filler, anyway.)
Compared to traditional roguelikes, Cogmind is slightly less reliant on the message log to convey important information, as combat animations are a big step towards transferring more attention to the map in event-heavy areas. Maintaining the right "balance of attention" is difficult, though, and there may not be enough reinforcement of certain combat results as is. More visual effects will be added to the map as necessary when development heads in that direction.
For now the message log is mostly for combat messages of the highest importance (like part/robot destruction), notices about changes in Cogmind's status (many of these are now accompanied by visual effects so the log or HUD data are not the only places to learn about them), and updates about what nearby neutral/unarmed units are doing.
The ultimate goal is to make the log a secondary source of information while the player focuses attention on the map. This is part of the reason why the log is so tiny, showing only six lines at a time!
Calling this "short" is only comparing it to your everyday multi-line log, because honestly games that reserve a single line for their message logs are ridiculously annoying.
If there are a lot of interesting events going on, or you think six lines is too few, or you just really like message logs, there is a new feature called the "extended log" available as a new mode for the multi-functional console first discussed along with the ally orders interface (so now this particular part of the interface has three options for what it shows). When the extended log is visible it shows the next six lines "older than" (above) those currently shown in the log:
The extended log, shown to the right of the normal message log.
The normal and extended logs scroll in tandem, so you essentially double the amount of information (12 lines) visible at any given time.
To see even more log messages, the normal log itself is now also expandable to the full height of the window, giving you at least 56 lines of log-viewing pleasure. Animated, of course:
Expanding the log is easy: Press F4 or click anywhere in the log to expand/shrink it.
A full-screen log would be more jarring to switch to (not to mention unnecessary since lines are generally short, anyway), so the log simply expands vertically in place. While expanded it's even easier to scroll back (via page up) to very old messages for references.
*Disclaimer: The messages seen in these screenshots are not representative of an actual game, just me fooling around in the debug playground to fill the log for demonstration.