The absence of the <p> tag in relation to forum posts.
I've noticed something with various forums. Well only phpbb and vbulletin. In the content of forum posts, they both don't use <p> tags for their paragraphs/content. They simply have it as <divs>. Now I understand this is because, in forums you may have tables and videos and what not, and you can't put them all into a <p> tag, and trying to differentiate what is paragraph and what isn't would be a nightmare in regex to put <p> tags between actual text and not images.
What I want to know is that, does this have any effect on SEO? Does it have effect on validation? Is it good practice to do this? And most importantly, is there a way to circumvent this? So forums and other applications would put <p> tag where it is necessary.