Well, here is why: The sketch I have is actually using a hardware timer interrupt as a tick and the main loop checks the tick counter and sends canbus frames out at the tick rate. The serial port is being used for diagnostics right now but the start up text is actually larger than the buffer. SO, what happens is that, on start up, the code blocks at the serial write function until I connect something to the serial port to get the data. Since it blocks, the main loop is not executing and thus the program is doing nothing. I could solve this by not outputting anything over serial until it receives something but it seems like the Arduino library should have had support for allowing characters to be thrown away if you don't care. For instance, it should be possible to run my sketch with the serial disconnected and just let all characters be thrown away. That's normally how serial output works. Making it always block just because nothing is connected is silly. I understand the reasoning (like you said, you don't want it to throw away things if you try to write too fast) but in my case that doesn't apply. It would be nice to be able to connect to the serial at any time and get new serial traffic without having to try to detect when something is listening. This would be super easy to do if I could rewrite the "write" method of HardwareSerial. I'd love to do that but the way Arduino is constructed does not make that easy.
I suppose my point is that blocking should always have been optional.