It can still be considered a rare occurrence when comparing it to the sheer number of guilds that exist in the game. With numbers well over 30,000 (going off of PVP ranks) we’re looking at at least 1,000 guilds. And if the bug happens to say 5 guilds, two times each, that’s 10 occurrences out of 1000 which is still only a 0.01% occurrence rate, which would be considered rare.
There is a Roster in the guild section that lists all your members. Manually counting those should give you a total number of members, rather than relying on the single number you mentioned.
I’m not sure how an online roster would alleviate the need. It would just be a copy/paste of what you see in game most likely, which would mean Guild Masters would still need to count them manually. The only advantage to having it on a web page as opposed to in game would be the community might be able to develop tools to help with tracking. Though (manual) guild tracking tools have been developed in the past to varying degrees of success, as every guild has different needs and requirements of what and how they track.
In the devs’ defense, the activity requirements for guild members is something the players themselves have imposed on the system, rather than being something built into the game. Yes it would be nice if the devs added more tools to help monitor, but the guild system was only recently reworked and we have Guild Wars coming on the horizon which will likely shake things up yet again. So the best thing to hope is that Guild Wars will also include better in game guild management tools.