There's many uses for random numbers where cryptographic random is unnecessary, but where different runs of the program should use different pseudo random sequences.
Of course, but in those cases, you should make sure that your initial seeds are random. It seems like in the older Go cases, the initial seed was fixed. But, if you wanted random numbers, you could have used your own (truly random) seed.
I don't really see an issue with either default, so long as it's documented and you can use a fixed seed if you'd like. I personally like needing to set a random seed explicitly. But then again, I learned about using random number generators a long time ago when this was always required, so what I think is obvious is probably a surprise to many (as shown by this stdlib change).
The only downside about this new change was that in the old way, if you needed a cryptographically secure random number, you had to explicitly call those functions from the stdlib. The choice should have been deliberate, but people don't like to read documentation...
