I've had a 2.5mm plug bend inside a DAP and completely rip the socket out when I went to unplug it. I've also seen lots of slanted 2.5mm plugs on cables out of the box, possibly because they're too thin to reliably make straight and QC properly. I've never had those issues with the 4.4mm connector, and I've never had a scenario where they were too big either. They fit perfectly fine in every case I've ever used, and they barely feel any heavier than the 2.5mm ones. It's not like you're suddenly carrying a 4-pin XLR plug on your IEMs. So, 4.4mm is definitely the way to go for me.
The fifth wire can be used for additional grounding with cables like Han Sound Audio's Venom. Whereas, if you were using a 2.5mm plug, you'd need an extra 3.5mm connector to connect to the DAP's SE port for the shielding to work, almost like a little tail hanging out the end of the cable. But, yeah, otherwise, it isn't really used I don't think. Again, the main advantage to me is in the durability.