Sub tasks don't work well in with Jira Agile.
In my current team we don't use them.
I disagree with sethcall, that there are no reasons that I should be using sub tasks.
I'd like to track that I can't easily at the moment:
- tasks for completing each point of a definition of done
- tasks to make acceptance tests pass
There are plugins that support acceptance testing, (e.g. Behave for JIRA) however I have no experience of them.
Some have suggested to use custom fields for your definition of done (and acceptance tests), but I'm not keen on that idea as there are too many exceptions to the list.
Here is an article that suggests using checklists for acceptance tests.
Might be worth a try.