I think that most "environmentally responsible" gardeners accept that controlled and selected use of weedkillers such as "Roundup", especially near paved/ uncultivated areas is an acceptable solution to the problem of maintenance of such areas. In the ideal world every weed would be pulled by hand, composted etc. I treat paving, edges of beds etc with a weedkiller every 3 months and have no qualms about it, it cuts down on maintenance, I only use the minimum amount of chemical required and it gives me time to focus on other things. Over time even the best pointing between slabs will break down, weeds will grow anyway, in the smallest hairline crack so weedkiller is the only option. As for salt, it's not good for concrete in the longterm.