Topeak's valve extender doesn't just add length to the outside of your valve, it also includes a captive rod inside the body that grabs the original valve's lock nut.
That means you can properly close the valve for peace of mind, though it has to be said it's rare for an open Presta valve to actually leak air.
Even if you usually use inner tubes with long valve stems with your deep section riims it's a good idea to have a valve extender handy in case you need to use a tube with a shorter valve. For tubular tyres you have no choice, as they only come with standard length valve stems.
The Topeak extender was too wide for our Corima rims, though we had no problems screwing it into Campagnolo rims.