For me, anything under 7 ft and a 4wt are a waste. But then I fish nothing but bamboo rods and silk lines... so what the heck do I know
The real shorty rods are so specialized that they will rarely get used unless you live in an unusual area.
My park rods are an 8ft, 5wt and usually a 7ft, 4wt depending on the stream I'm on.