WordPress is an open project and therefore there are a lot of people all around the world developing extensions (plugins) to it.
Plugins do make life easier for the blogger, allowing new features addition and exploitation.
However, the question arises: how many plugins should actually be installed?
It ought to be taken into consideration that every single plugin requires some work on the part of the blogger: testing, installation, tuning, updating, etc.
For the same plugin, updating and re-tuning must be carried out on every blog it is installed on. So, if for instance you manage 20 blogs and each plugin update/retuning takes 10 minutes, the overall time needed is 20*10 = 200 minutes, that is more than 3 hours for that same plugin.
If you usually work with 10 different plugins on each blog, then the required housekeeping time can skyrocket to 200*10 = 2000 minutes, i.e. over 30 hours.
Clearly, frequency and duration of plugin updating are extremely variable and sometimes updating can occur only rarely.
Anyway, it does occur and consequently it could prove advisable not to resort to too many (different) plugins on a high number of blogs.
