In applying for ESA and PIP Care Plans are mentioned in terms of medical evidence you can send in. I heard recently that only 5% of people with a long term condition actually have a care plan, although everyone who has such a condition is entitled to one.
Going through the applications process myself, I noticed the care plan was referred to in assessing me for benefits. I would be interested to hear the vies of the Mods, or other patients who have used them.
In comparison with paying for doctor's letters, which may be costly and sometimes seen as biased by the DWP the benefits of care plans would be they are free, can help you manage your condition and be used as evidence of how your condition affects you.
For people with mental health conditions, the recovery planning / programme can be very helpful.
Here is some more information about Care Plans
www.nhs.uk/Planners/Yourhealth/Pages/Careplan.aspx 
