It's true that a M4/3 sensor is smaller than the APS sized Fuji, M4/3 ends up with a crop factor of x2. The Olympus 200mm ends up as 284mm F2.8 equivalent on my G7. This is a lens I already own and for the sake of an outlay of £109, I can use it on my camera whilst taking into account that manual focusing will be required and gain a stop of light. I could, of course, just buy the Olympus 40-150mm F2.8 Pro for nearly £1000 or mount a Canon 300mm F2.8 onto my full frame Canon DSLR for £4500; this would enable autofocus and undeniably better image quality. So for me, the £109 was a relatively small investment to gain the extra stop of light and to have the pleasure of using my older generation lenses, which provide a different look but likely lower image quality. I'm clearly not alone in this area, the prices of certain older manual lenses are rising quickly on Ebay. My recently acquired Tamron F2.5 Adaptall Macro Lens gives stunning results on the small number of test shots so far.
The addition of optical elements in an adapter could result in image quality degrading and this was certainly true of the Lens Turbo Version 1 and the infamous blue spot. Later versions don't seem to have any issues, especially when using the very costly Metabones. Tony Northrup has done a number of videos on adapted lenses and adapters, most of his recent videos are shot entirely in 4K with speed boosted GH4s (and probably EF lenses) without quality issues.
Perhaps the real question is why do some photographers want to adopt lenses and go to the faff of manual focusing and using a heavy old lens instead of a lightwieght modern lens. Some gain pleasure from doing so and others think it a waste of time, each to their own.