Why should I not separate alloc and init?

Question

The normal way to initialise and allocate in Objective-C is

NSObject *someObject = [[NSObject alloc] init];

Why is the following not practised

Answer 1

From the Object Initialization official documentation:

Because an init... method might return nil or an object other than the one explicitly allocated, it is dangerous to use the instance returned by alloc or allocWithZone: instead of the one returned by the initializer

Another reason from the same document is that:

Once an object is initialized, you should not initialize it again

given this example:

NSString *aStr = [[NSString alloc] initWithString:@"Foo"];
aStr = [aStr initWithString:@"Bar"];

where:

the second initialization in this example would result in NSInvalidArgumentException being raised.