Notes are not searchable. Notes cannot be included in reports or advanced finds.
Many organizations like to have statistics and reports around what activities have been completed.
It is often recommended to NOT use notes in Dynamics portal because they do not roll up. Notes on a case, for example, are not visible on the account and vice versa. This might work well because notes on a case may be temporary and not critical information that needs to be shared with others. However, when using notes for everything, it's possible for critical information to get "lost" because they are kept where others may not see them.
Another reason to not use notes is the user may have more than one place to go in order to see notes. For organizations that standardize on keeping all notes on the account, for example, the users still have to look at both the activities and the notes. Heaven forbid users are keeping notes on multiple different records, multiplying the number of places to go to see what is going on!