Alan Bleiweiss: Proper internal linking is important for maximizing SEO. How important is based on all the other signals.
Any one signal can be needed less if enough other signals get the job done.
Yet it always comes down to visitors. How well you provide intuitive access to your own content will determine how effortless or difficult it is for visitors to not only get to the content they want upon arrival, but also how long they stay and explore other content.
So while you may be able to "get away with it" with poor internal linking for SEO, that is a high risk model and you may be missing out on significant opportunity.
Neil Cheesman: There is an interesting `study/report` on the world cup and daily mail (UK newspaper) failing to rank for world cup a few years ago - interesting reading - search Google for "world cup internal linking daily mail" - just says about the importance of linking to the relevant page and not cannabilising with other pages for the same term etc... and of course, knowing your customers and what they are likely looking for.