I agree with Dale - I usually only put quick notes in task notes: websites, email addresses, phone numbers - things that, by not having handy, may delay me when I want to complete the task. Evernote is much better for lengthy notes that you may want to reference and keep around beyond the lifetime of the task. The task note will essentially disappear once complete.
What I have found useful, however, is accruing notes on my repeating tasks: information, quotes, or other little motivators to keep me completing these frequent tasks that may get boring. For example, I have a monthly task to calibrate my MacBook Air battery, for longevities sake. Inside the task, I have the following note:
For optimal battery life, it is vital that you 'condition' the battery regularly. Once every 4-6 weeks, ensure that the battery is fully charged (97 to 100%) and then disconnect the MagSafe, and run it until you receive the 'low battery' warning. Shut the machine down, then reconnect it to the MagSafe and do not use the machine until the MagSafe's charge indicator LED turns green.