In bug 14055, the decision was to add functionality for XF86Battery control in the power manager and believe this wasn't the best course of action as it adds UI functionality that most users wouldn't use. Instead it should have been added as a keyboard shortcut, which could easily be modifiable to the user to set what functionality is wanted.
I presume the default behaviour of the keyboard shortcut should be to open the Power Manager and switch to the Devices tab and ideally selecting the battery entry.
Actually it's quite hard for users to set a keyboard shortcut to "suspend" because xfce4-power-manager doesn't expose a cli for such things, so the user has to know his/her distribution well. Regarding your other point, yes, that would be another option, to open the settings dialog with the devices tab. As you can read from the original report the Battery key used to trigger a status notification showing the status of the battery. This would be nice to bring back, but I personally have no motivation to work on tat. I think the current use case (e.g. being able to call suspend etc) makes sense as well (obviously I think that, otherwise I wouldn't have merged the patch).
Not sure why XF86Battery would need to be set as 'suspend' when it was never the function of the key to do suspend or hibernate type functions. It would be like me suggesting any arbitrary function key, like XF86WWW, to be allowed this same functionality and having it visually exposed in the power manager UI. If such functionality is wanted to be kept and controlled by power manager, then it is fine to keep it as is until a patch can be provided to open the settings dialog, but I would suggest that its visual presence in the UI be removed/hidden (which I can send in a patch for) and those that wish to adjust this functionality to open up settings editor to do so.
As long as you select "Do nothing" you can still bind keyboard shortcuts to it, so no, I don't agree that it should be hidden.
As we also handle all other power related buttons we will also keep the battery button there as it can be easily "unhandled" and set to any other keyboard shortcut by setting it to "Do nothing".