Random vs RandomNumberGenerator [duplicate]

Question

Why would anybody use the "standard" random number generator from System.Random at all instead of always using the cryptographically secure random number generator from System.Security.Cryptography.RandomNumberGenerator (or its subclasses because RandomNumberGenerator is abstract)?

Nate Lawson tells us in his Google Tech Talk presentation "Crypto Strikes Back" at minute 13:11 not to use the "standard" random number generators from Python, Java and C# and to instead use the cryptographically secure version.

I know the difference between the two versions of random number generators (see question 101337).

But what rationale is there to not always use the secure random number generator? Why use System.Random at all? Performance perhaps?

Answer 1

Speed and intent. If you're generating a random number and have no need for security, why use a slow crypto function? You don't need security, so why make someone else think that the number may be used for something secure when it won't be?

Answer 2

Apart from the speed and the more useful interface (NextDouble() etc) it is also possible to make a repeatable random sequence by using a fixed seed value. That is quite useful, amongst others during Testing.

Random gen1 = new Random();     // auto seeded by the clock
Random gen2 = new Random(0);    // Next(10) always yields 7,8,7,5,2,....