crucial is a strong word
id say its important, and depends on the type of content youre uploading
if its in an oversaturated market, i think social media can breathe some life into your channel and possibly revive it
if youre in a more niche market, or youre catering to a newly developed market, i dont think its necessary
but when it comes down to it: if your seo is good, you got an awesome channel with awesome content, and a competitor emerges, and hes using social media to reach out to more people - why wouldnt you do it too?
either way, youre letting go of an advantage that will probably benefit your channel greatly, or have a small, but positive impact