My friend is doing a "teaching assistant" course, its just 2 days college work a month. She is unemployed and a single mum to 2 dc at the school. Yet she works at the school nearly full time as a teaching assistant - but doesn't get paid anything for it.
And I recently found out there were four other mums who do exactly the same as well - unpaid TA volunteers - and thats in DS's year alone. 
I have no problem with my DC being helped by these mums, they are lovely and I am sure they are more than capable. And of course its great there are lots of staff around to help the DCs.
But I think its a bit unfair on the mums that volunteer tbh. If they need FT TA's they should be paying them. I mean, fair enough, you could argue they are gaining valuable experience to try and move into a paid TA role, but my guess is there will be next to no roles available anyway, as no school is going to be employing new TA's when they can get them for free? I dunno, it just doesn't sit right with me, smacks a bit of exploitation.