Agree with nearly all the posts. For sure HF will get you sessions on days where you might otherwise be skunked, so it will get you on the water more. But another big factor for me: if your time within a session is limited (e.g., sometimes I have 2 hours total), then bringing a combination of HF, surfboard, and twintip will massively expand a given kite's wind range, so if the wind changes during the session you don't need to re rig a bigger or smaller kite. Plus it's fun to mix it up, keep different muscles moving, keep the stoke up.
The one thing I don't agree with: "How many times have I said " shite, wind is too strong"
I initially started foiling for light wind, but it's become way more fun in higher winds, with the wind swells that result. The perfect session for me is mid to high 20s on on my 6m. Though 30+ I'll agree, gets to be a handful to control all that speed.