If you delete the page, and it had links pointing to it, the server will return a 404 not found page, which makes you lose any authority they had to pass to the main domain or subdomain.
Using the 301 redirects at least you take a portion of the authority back to your Website. If your CMS is somehow advanced, it should be easy to hide those "expired pages" from the page list avoiding any confusion.
But again, if you redirect the page, lets say about am inbound marketing conference in Boston to the main domain that does not "serve" any kind of useful content to the user that was actually expecting the page of the Boston conference, that won't help at all. Instead, try to 301 them to something that the user may be interested in, even tho the event he was looking for is no longer available; in this case, it could be a page listing all the upcoming inbound marketing conferences (in/near Boston). By going that route you favor your site by making the pagerank flow to the other page and you also help the user, which is the primary target.