Plus, the display of the Serene logo masks the time it's taking for the program to be loaded up on the computer. Even if there were no logo, the program would still require those few moments to start, so it's better to give people a pretty logo to look at while that happens.
(Don't forget that this is actual an aquarium simulation program which just happens to have the side benefit of also working as a screen saver; it's not just a screensaver purely and simply.)
And hey, if Microsoft and Adobe and Jasc and all those people can get away with those splash screens with no adverse reactions...
