I've done some volunteering in the past and whilst it was was useful to me on a personal level (ie getting me up and about each day), no employer or agency I've applied for work with has been interested, I just get the usual 'and what about your last full time paid position?' question asked whenever I try to emphasise any useful activity I've done since becoming unemployed.
I finished with Ingeus last September and was sent for a MWA in November/December. I have 12 years clerical experience plus a number of clerical and IT based qualifications that I've achieved both in and out of work, and I was placed with......a 2nd hand council furniture store. Basically spent my 4 weeks doing nothing useful to my chosen career path, including some moving heavy furniture (and putting flat pack furniture together) and cleaning, despite both the JCP and the MWA provider (LearnDirect) knowing that I am actually limited in the amount of physical activity I can do (and that I actually receive low rate care/mobility DLA)....luckily the people at the placement were pretty reasonable, quickly realised my limits and didn't expect me to do anything I couldn't do, and I even ended up with a good reference from them.
Since then, I've had no interviews and barely any replies from jobs I've applied for, asides from automated acknowledgements of applications, and the agencies that have bothered to get back in touch with me aren't interested in the MWA at all as it has zilch to do with any of the clerical/admin positions I apply for.