Is it always necessary to source manufacturer’s own roof boxes?

Roof boxes are an increasingly must-have addition to one's vehicle these days. Cars (as the shift from family "car" to family "MPV" or "people transporter" has shown) are less about popping to the shops, or driving to and from work, as they are about making some kind of leisure statement. Stuffing the whole family in and throwing a bunch of sporting equipment on the roof. Taking elaborate days out where more clobber is required than can be fit in the boot. And, of course, facilitating the adventure sports that half the nation seems to have embraced as its hobby of choice.
Roof boxes can be (and are) filled with surf boards, mountain bikes, kite boards, power kites: name it, and there's been a roof box somewhere in the country that gets regularly filled with it. All well and good, and very healthy, but what about the car itself? A lot of car manufacturers produce their own accessories, roof bars, boxes, bike racks and so on - which they claim "should" be fitted to their vehicles in order to preserve warranty, ensure the correct fit and so on. Is this wholly true?
In a very short word: no. Not really. It is true that most major vehicle manufacturers produce their own roof boxes, which can be fitted with ease by a local dealer of the marque. It's also true that own-brand (as in manufacturer's brand, not store brand) roof kit will be protected by a warranty that is bolstered by the fact that the gear in question has been specifically designed for that vehicle. But that's about as far as it goes. The major makers of roof bars, bike racks and so on produce high quality, fully guaranteed luggage options and accessories, which, if professionally fitted, are just as protected under warranty as car manufacturer's own. In many cases, these car luggage brand roof boxes are actually better protected, precisely because they haven't been made by the company that makes the car. All good car luggage options are sold "to fit" specified makes and models of vehicle - therefore, if they are professionally fitted and they damage the vehicle in some way, their warranty will cover repairs.
Generally speaking, own brand (i.e., non car makers') roof kit is cheaper - and, in a lot of cases, actually better designed for its purpose. Sports roof boxes in particular, which are obviously intended to hold specified items of sporting equipment - such as surf boards, mountain bikes or wind surfing gear. A person cannot put wind surfing gear in a roof box not designed to fit it - it's of too awkward a shape. Sports roof holders will be made to protect the gear inside as much as the car outside - and that, of course, when one is maintaining a hobby with such expensive kit as wind surfing, is invaluable.
All in all, then, it does not necessarily is required for a person to listen to the often plaintive claims made by a car manufacturer that only roof boxes made by them should be fitted. The right roof box is the one that holds the right amount of stuff, safely, for the right price - whoever it might be made by.