Actually, the production Ducati Desmo 250, 350 and 450 singles came standard with quite strong hairpin valve springs. On mine, I can just press them down with my fingers to release the spring retainer. The closing rockers can be modified to take much lighter 'helper' springs as fitted to the V twins from the 70s to the 90s and onwards. This is what I have done (with thanks to forum member GRM 450). A long time ago, I ran the motor without any springs and it could be started. If the closing rocker clearances are too large, then it might be tricky to start. Once started, gas pressure slaps the valves back fully onto their seats.
My experience of desmos is that the valve clearances in the old 70s engines need very frequent adjustment, and because of cam grinding inaccuracies, it is better to remove the heads to set the clearances to avoid rocker/cam binding. It is a lot of bother for very little if any practical return. The top Ducati tuners have proved that the valve spring engines are just as effective as the desmos and much less hassle. The modern desmo Ducatis are built to much closer tolerances and from better materials, resulting in much longer service intervals without having to remove the heads to set the valve clearances. They too have light 'helper' return springs. The engine efficiency arguments mentioned above are interesting, but equally, Ducati use their desmo system as a marketing tool to very good effect because they are still the only production machines using the system, so it has given them an aura of exotic mystique.
The desmo system had another advantage back in the day, when brakes were poor and the revs would soar on the overrun using engine braking, there was no risk of valve tangling.Ohv pushrod owners, consider yourselves lucky you don't have to maintain a desmo Ducati. It's a real luxury to be able to whip off the valve covers, slacken the lock nut and turn the adjuster against the feeler gauge. Job done, and then you are back on the road again, which is what it is all about.
