the drawbacks compared to what? compared to not optimizing your website for
SEO?
if you want your website to get found by searchers, your choices are to a) run a ppc campaign, b) optimize your site for organic rankings, or c) use both ppc and
seo to market your site.
i choose option C.
and btw, if you do your keyword optimization correctly and conduct extensive keyword research and constant keyword discovery, you'll never overlook the important keywords. in fact, the opposite it true. by conducting keyword research, you will discover the important keywords.
good luck.