Unless you could do overnight pcb production with quantity 1 boards in the $1 range, I don't see how custom pcb's would work during development. Sure, once you have the basic design down, then I can see the use of custom pcb's.
Well, playing devil's advocate to myself from earlier in the thread, there ARE moments when custom PCBs are necessary. Since my latest version of this device is almost entirely SMT, there were a couple components that just... didn't have a breakout board. At all. Namely, my cypress USB hub chip, and my FPC connector. The USB hub doesn't have a hobbyist-friendly breakout board for a pretty good reason, which is that a lot of the PCB design specs are pretty stringent (minimum 4-layer board, and USB data line length matching being the major ones), and the FPC connector was an odd size. Simplest way for me to test designs for those components was to make a test board: I just made a one-off board from oshpark that had a few USB connectors, a built-in Teensy 3.1, and the screen connector. Tested the USB hub by making sure that the teensy appeared on my computer, and any device I plugged into the extra usb connectors worked, and tested the FPC connector by having the teensy run some TFT touchscreen code.
Basically would have been out-and-out impossible to try and breadboard everything and expect any kind of real success, due to all the PCB-level design requirements for those components.
Anyway, I suppose that doesn't really contribute to the thread much - no hobbyist wants to deal with that level of headache, I'm sure. (The only reason I did was because NOBODY made a breakout for that USB hub chip. I plan to get working on one myself in the next few months or so, actually).