I personaly wouldn't have a horse that couldn't hack out alone - that said, it's not worth it at the expense of your safety if a horse won't hack out a lone!
I think all horses should be able to hack out alone, not least because you wont always have company when trying to keep a competition horse fit.
I have one at the moment who is far better hacking out alone then in company. Alone he is a total star, wont blink at anything and walks past tractors as though they were not there.
get him in company and he's a totaly different pony. Whilst he isnt dangerous, he does spook at marks on the road, he does get strong and tractors suddenly turn into pony eating monsters that require me getting off and leading him past. My friends are convinced he plays to the crowd, he's always been like that though and when you get him in the show ring the bigger the crowd the more he shows off.
I believe it's absolutely important that horses can and will hack alone, even if they rarely have to do it for those of you that keep horses at livery yards.
I keep my horses at home, alone, so of course it's a must for me anyway. Even if I meet up with someone out hacking, I have to get to the meet up point alone.