> I see many posts about I2C.  Not so many SPI queries.
> This leads me to suspect that I2C is more popular than SPI.
> Why is this?

        I don't think this is necessarily so, I believe you see more posts on I2C
because it is more complex and problems pop up more frequently. The SPI
device can basically be treated as a shift register with enable, VERY simple
to implement with a microcontroller or a PC. I2C on the other hand is a bus
with many devices on it, addressing is present, inputs bus lines are
bi-directional and a master sometimes isn't a master anymore (on a
multimaster bus). Frankly I think SPI is easily as popular as I2C, in fact
some manufacturers have barely any I2C devices (NS comes to mind, AFAIK most
of their serial devices are SPI). Perhaps it has something to do with
licensing, I don't know. TTYL