yes and no
in theory i get what you're saying and it hadn't occured to me at first, after a few minutes deliberaton, i've came to the following conclusion:
firstly, i dont think shine Dj FF dair OOS is BETTER than wavedashing out, but for all intents and purposes, it doesnt hinder your combo ability because you can combo quite well out of it, so being able to waveshine isnt a necessesity is what i was getting at
second, waveshining is crucial for following up when your opponent gets good SDI on your shines. DJing kills your horizontal momentum pretty significantly so its difficult to chase di if you always jump out. The difference is, when you're combo'ing someone, they have ample time to input proper DI because they KNOW exactly whats coming. you dair'd/nair'd them, your next move is shine, they can prepare for it really well.
shining out of shield is slightly differen, you often do it in situations where your opponent is directly on top of your sheild, probably in the process of fastfalling (terrible DI input) or perhaps crossing you up with a flyby nair or something (again, not really that great DI inputting on their part). so you often land shines oos in situations where is very easy to DJ and land a falling dair into more shines/uptilts
anyway, now im starting to ramble. this is all from experience and watching vids. Not taking anything away from waveshining oos, thats obviously a great prospect too, but shine dj oos is one of those things as falco thats really easy and provides many of the same oppotunities as other "better" options.