Always an interesting one this. I too deal with risk professionally and am often asked to weigh possible impact and likelihood against the cost/effort involved to reduce that risk significantly. I don't, I should point out, have any statistical model for this however it seems to me that a good glass or metal insert significantly reduces the risk of a catastrophic fire. Admittedly that risk is also mediated by the quality of tealight used (very cheap ones DO spit and smoke), user judgements, ignorance, environmental conditions, chance and the quality/fire safety of the rest of the built environment amongst many other things. They are inexpensive and generally improve the aesthetic of tealight holders (can be concealed if needed). I have always used inserts. Just one of those things that makes sense to me. Sorry if that sounded like a rant, I did knock off work an hour ago, merry crimbo all