If you create a blog, and work on it, eventually it will rise in value. It may even get a Google Page Rank of some decent value.
So let's say you manage to make something of your blog, then tire of it. The normal thing would be to delete it. Delete it - it's gone - move on to something new.
As RedRyder52 at Blogger Forum points out, you'll just be creating an opportunity for yet another spammer to move in, and to capitalise on your success.
And here's another experience. AS had his blog deleted (not by her), and before she could reregister the name, it was stolen by a bad guy. Fortunately, with some hard work, patience, and persistence, her experience had a happy ending.
If you decide to switch to external publishing, so you can use your own domain, you'll be doing the same. You'll give up your blogspot.com address, if you don't plan for the move to your new address.
The bad guys are out there, with automated tools. They can tell when a previously existing blog, particularly one with any value, doesn't exist any more. Within seconds of its deletion, they may be able to register it in their name, and put up a small post or two pointing your former or wanta be customers to their website. The value of your blog becomes theirs, simply because you did not want it.
So when you tire of your blog, if you have ANY compassion for the folks who might do a search on any keywords that are included in the blog right now, don't delete it. If you do, the spammer will love you, but the folks who search for help and get the spammers page, based upon YOUR hard work and success, won't.
Also, some folks have a saying "The Internet is forever", and that saying surely applies here. Some search engines may reference the cached copy (from the last time they scanned your blog) for a long time, if you just delete your blog. Don't delete your blog, unless you can plan the deletion process, properly.
If you're going to switch to external publishing, Please! Do your readers (and yourself) a huge favour, and plan the move carefully!!
Here's another caution. Mistakes happen. If you made a mistake, and deleted your blog, plan the recovery properly.