Do I really need to see that I'm still running TextExpander for shortcut handling, or that I'm still running ScreenFlow for recording screencasts, every single time I press Command-Tab? Absolutely not. By keeping these apps in the menu bar, my Command-Tab switcher contains only the programs I actually want to switch to and from. Segregating my "helper" utilities to the menu bar (when I can) leaves the "real" applications for the Dock and Command-Tab program switcher.
Menu-bar icons are not always a perfect solution, however. One big usability issue is that you can't easily rearrange the program icons in your menu bar. (You can rearrange some menu-bar icons by holding Command and dragging, but this technique is reserved for Apple's own apps.) The order of the icons on the menu bar is based on the order in which the apps launched, so you have to quit and relaunch your menu-bar apps in the right order if you want to achieve a specific arrangement in the menu bar. If you then quit and relaunch one of the programs, the order changes again.
Another usability issue is that you're never quite sure what will happen when you click a menu-bar icon. The majority of menu-bar icons open a drop-down menu when clicked, but some display an attached window, while others open a standard Mac application window. The inconsistency can be jarring.
The last problem is bloat. On my 27-inch iMac, I presently have 14 apps with menu-bar icons running. And although that isn't a problem with 2560 horizontal pixels available, it's a different story on my 1366-pixel 11-inch MacBook Air: Running a similar set of menu-bar apps there leads to the loss of all available menu-bar space.
As more utility apps offer the option of putting an icon in the menu bar, bloat may become an issue on my big iMac, too. Even if I don't run out of usable space, having 25 icons lined up across the top of the screen is something of a usability nightmare.
While Apple has provided no official fix for this issue, one third-party app--a menu-bar application, naturally--not only solves the space problem but also corrects the arrangement issue: Bartender allows you to consolidate all of your menu-bar app icons, including the Apple-provided ones, onto what you might call a sub-menu bar. You can easily rearrange items on the sub-menu bar by Command-dragging them, and you can choose to have apps that indicate changes by modifying their icons to appear temporarily on the main menu bar when their state does change.
Here's what my previously overcrowded MacBook Air's menu bar looks like with Bartender installed and set to consolidate just the third-party menu-bar icons:
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