tlgosser - "f you really need speed the best way to do it is usually with a parallel input DAC."
Agreed, if high speed were the only aim.
The data rate for audio is quite low. Christopher “Monty” Montgomery has quite a good explananation of why 16 bit 48ksamples/s is as good as we can perceive: http://xiph.org/~xiphmont/demo/neil-young.html
For mono that is only 48k * 16 = 768Kbit/s.
Even if we go for higher sample rates, and wider samples to make signal processing easier, then 192ksample/s 24bit/sample or 32 bit/sample is a common serial data rate, AFAIK good enough for most applications. For stereo that is 2 * 192k * 32 = 12.288 Mbit/s or 768k 16bit transfers/second.
This is well within the capability of Maple's on-board serial shift registers (upto 18Mbit/s). This is about the same as USB. So if the signal paths to the DAC or CODEC are reasonably short and shielded, it should be pretty good.
Further serial transfer only takes three pins, data, clock and 'word select', which is much easier to layout than 16, 24 or 32 pins plus latch and right/left. Also SPI or I2S may have a small benefit of flexibility to trade-off sample rate and word size without changing board layout.
Much MCU and chip pricing is strongly related to the number of pins. So for production, a chip which only needs 3 pins for audio vs 17 upto 34 pins will be lower cost, and potentially require less PCB area. So bit serial has some useful commercial and technical advantages over parallel data transfer at these low-bit rates.
I2S is an augmented SPI. As well as SPI's data and clock signals, it provides 'word select' which is typically used to identify the channel, and also defines the word length or number of bits in a frame. The TDA1543 is a low-cost example, it is an 8pin chip, which is likely part of the reason it is so low-cost. AFAICT, the majority of consumer-end audio CODECS use I2S for audio data.
There may be another benefit from serial audio data, especially most-significant-bit first. The DAC might not actually implement a traditional DACs multi-value conversion mechanism, but instead might be 'bit stream' or 1-bit DAC: http://en.wikipedia.org/wiki/1-bit_DAC. Bit stream conversion can be easy and cheap to make. One advantage over multi-value DACs is the output signal 'runs' at multi-MHz; the the noise from D to A conversion is such a high frequency that it is easy to filter from the audio output. Audio CODECs often include this circuitry, making them even easier to use than a simple DAC.
If folks used the more recent STM32F's (e.g. STM32F3 on my Orone-mini :-) or the higher-density STM32F103 (e.g. on the RET6), then they get I2S peripheral hardware on the MCU, so this becomes even easier.
(Full disclosure: I am not a member of LeafLabs staff.)