I have a dual sim phone and still hardly use the data, but it's handy to have in emergencies.
Just don't use it for data.. 4Gb will cost you £40
I too have free 200 mb data from three but I hardly use it as I don't have a dual sim phone. I use my sons three payg phone when calling from abroad to non - UK numbers as it is cheaper than my contract phone.
I'm trying to get mine to come to o2, I changed because I hardly use my phone, I always had loads of credit and was out of the 3 months, so thought I would switch to a cheaper rate. My parents still like the free giffgaff to giffgaff calls. My mum always tops up before 3 months, sometimes 2, my dad doesn't though. They could use the data whilst out too for whatsapp, 1p is better than 5p per MB! I can use my spare credit for google play apps too, something you couldn't do on giffgaff.