I know many will disagree but I just want to say 2 things:
1) yes, horses have very good eyesight in some ways (like, can notice a fox trotting across next field half a mile off at dusk). However they have rather *poor* eyesight in certain other ways, mainly a) objects very close to their head, b) 'fine detail'
non-moving objects, and most especially c) anything not hugely conspicuous while they are running along being silly or being afraid. Sadly, (a) and (c) are the cause of a lot of crippling or fatal fence injuries.
2) it is certainly true that a sedate horse will often stay within pretty much any kind of fence and not get hurt. I mean, for a horse that doesn't want to go elsewhere, *string* (literally, and I don't mean electrified) will work 98% of the time; and look at how many horses live in barb wire paddocks full of rusty buicks and tine-harrows and haven't got hurt.
Yet. But "yet" is exactly the problem. A horse's chances of getting out and/or getting hurt ARE demonstrably much higher in some kinds of fences than in others. You will not notice this if you own few horses or have only had horses for 10 years, but if you work with (or know) LOTS of horses and long periods of time it becomes strikingly obvious.
There is also the consideration of what's at risk. If nothing too awful is likely to result from an escape, fine, use something less-visible if you must, and/or flimsy - in fact flimsy is probably smart in that situation as it makes 'paper cuts' more likely than slicing thru tendons. But if you live on a well travelled road or near some other significant hazard, it may be more important to keep the horse in no matter what, and for that, one or two strands of *anything* is just a really poor bet.
The thing is, an awful lot of really grievously horrible fence injuries, and escapes as well, happen to horses who have lived in that fence for a long time. Maybe some loose dogs chase the horse one day, maybe a hot air balloon lands nearby, maybe a fire engine shows up, maybe the horse just has an unusually wild 'fun run' one fine day - and bang, you've got a big problem. 'Nothing bad has happened yet' does not necessarily mean 'smart choice, low risk'.
It usually does not cost much more to construct a safer fence *for your particular situation* (there is no universal 'best way') than to construct a more-dangerous one. Indeed, the safer option is often no more expensive, sometimes even cheaper. I think the goal should be to never really know whether it was worth it - that is, to never
have the kinds of vet bills or euthanasia that make you go 'oh jeez, if only I'd'.
It's good to learn from others' experience rather than insisting on making all one's own mistakes
-- but with horses, it's other peoples' *negative* experiences that probably have the biggest lessons for us.
Sittin' down and shuttin' up now,
Pat