What I know about using numeric values in permalinks is that it reduces the chances of clone permalinks to happen. What I've generally seen for wordpress blogs is the use of the date (such as the year and perhaps even month) followed by the postname, such as /%year%/%monthnum%/%postname%/, in order to avoid clone post permalinks should two posts have the same title.
Perhaps what you referring to aswell is the use of the post's id instead of using the postname, such as /%post_id%/, where you have 0% chance of having two posts with the same permalink since IDs are always unique.
Either way, WordPress will increment the permalink if one's already used so you never will have an error of two posts using the same permalink. Problem is that having your-post-permalink-2 isn't the most beautiful name, because of the prefixed 2 at the end (if there's two posts with the same permalink). That's why including a temporal variable such as the year or month will reduce the chances that you'll have two identical permalinks so they all remain as 'organic' as possible.
I've checked on my own sites. I use /%category%/%postname%/ for my posts. For my pages, the permalink structure is reduced to /%postname%/ because pages have no categories. So if you switch over to category/postnames, your pages permalinks won't be affected, so your external links won't be broken.