[Rover]

Front glass is usually not a thin slab of easily breakable glass, so a broken filter is not an indication to what would have happened to the front element, though. Personally, I do not like the idea of sharp shards of broken glass scratching the front element, so I am not someone to put filters on as standard protection. Thin slabs of glass just do not provide much protection other than touching.

 

Some tele lenses really do not like protective filters, for some reason they shoot OOF with them. I have seen a number of posts complaining about that, but I do not know why it happens.

I see where you're coming from, but a badly scratched element is as good as useless anyway - even if it's not outright smashed. Both times I've had the front filters demolished on my lenses (once on a 24-85 and once on a 70-200/2.8 L IS) the front elements came to no harm. Remember, something that attacks the front element may be harder than glass, so it may inflict more damage than the glass shards would do - even though the latter would certainly dent the coatings.

Besides, some lenses are stated as requiring front filters to complete the sealing. The 16-35/17-40 class lenses are among these. A 16-35 happens to be my most used lens so I would not expose it without necessity.