, you're right. If you run different widgets in each launcher, a quick look at what's running will show them all running. But there's also a piece of the launcher that tells the system it's what will show on the homescreen. Resetting default or just switching launchers doesn't always take that out of memory.
I use Action Launcher Pro and Aviate. If I don't reboot, I can switch launcher (including which is default), open an app, close the app to find myself in the launcher I switched from, despite the fact it's no longer the launcher of choice or the default. So, I just got in the habit of rebooting to clear memory. It works.
And you're right about not needing an app to change defaults. I assumed by the question +Andi Robinson
asked that she might not realize it was a system setting. Homesmack, which also can change the default for search and browser, is easy to use for that. Default App Manager Pro does the same for a bunch of other default settings. For the novice it's easy, for the more expert user, it saves steps.