Azrael360: At least, I can confirm that in Serious Sam 2 is obligatory to use that client.
SirPrimalform: Actually, SS2 is among the very small number of games on Desura that are just a Steam key so the Desura client definitely isn't required for them (but the Steam client is).
LOL, you're right! :P
I own that game, but I haven't played it (and installed it) yet... It was part of a bundle, but I'm not so interested in playing it.
EDIT:
Actually, this is the message when I tried to install the game from them:
"You need Desura to play Serious Sam 2 (view standalone downloads)."
And if I click on "standalone downloads":
"Desura is a digital distribution app which allows you to install and play games with one click. Serious Sam 2 will be available to download soon on Desura.
Contact us, if you would like us to prioritize adding a standalone download for this game, that doesn't require the Desura app to install."
So yeah... :P