The one "advantage" I don't see is:
Many apps won't move to the SD card. Of those that will, not all will work. (Which is why many won't move - the developer knows his app won't work from the card, so he doesn't allow it to be moved - the Move to SD card button is greyed out.)
Of those apps that will move to the card and work from it, not the entire app will move. At very best, since Android's still looking for the app in internal storage, there's a link saved where the app used to be, pointing to where it is. So for small apps you're not saving as much as you thought you would be. And if the app is written in multiple pieces, each piece gets moved and leaves a link behind (because the app is looking for its other parts in internal storage) so, if the app is written in small enough pieces, you take up more internal storage with the app on the SD card than you would with it left in internal storage.
Android was never designed to have apps run from the SD card. (Widgets, which are just user interfaces to apps, won't and never have been able to.) It was designed to use the external card for data storage - pictures, videos, music, documents, etc.
So, depending on your particular situation, you may or may not even be able to run apps from the card, and if you can, you might not save much space.
Also, cards are designed to be written to a limited number of times. When a "sector" fails, the card has to be reformatted. Internal storage is more like an SSD - it's designed to distribute writes to prolong its life. If the app is writing its data to external storage constantly (Android apps can be killed by Android at any time, so they're responsible for storing any change that they want kept when they're reloaded later. ("Later" in this case can be microseconds.) If the app isn't constantly saving its data, you may be playing a game and be on level 9, and all of a sudden you're back on level 4 - the last time the game saved data. Oops. Constantly writing to an SD card isn't good for the card's life.