@Chris 17. There are a few reasons:
1. I got a 5Gb/month mobile broadband SIM from Three on a half price deal, so that's £8.50/month. I mostly use it for tethering, and use about 4Gb as it is shared between my phone/laptop and the kids (when we're in the car). If I cancelled that I wouldn't get such a good deal again.
2. I have had various voice/text SIMs as needs have arisen - Virgin, Ovivo, GiffGaff. So I can port the number everyone knows to whichever company gives the best deal for me. I hardly use any minutes, and what I do use tends to be done via VOIP over 4G (which EE block). Monthly spend on voice/text is around £0.20.
3. As hinted above, some companies block ports or NAT internet access (Virgin, EE). Three appears to be completely open so is great for VOIP as well as VPN and the usual web surfing. The signal has improved massively in the past few years on the routes that I take, so I am reluctant to move. O2 (GiffGaff) sucks for data esp. 4G around my area.
So the only limitation I have is that I can't use Three for both SIMs, for the reasons mentioned in my previous post.
BTW, thanks to the keyboard warrior for the downvote, care to share why?