So, something like the GDS-2104E to get 100 MHz and 4 channels - $1200.
That's a little over the top for me and my little hobby. Actually, the DS1054Z was a stretch since I already had a Tektronix 485.
Still, if that decoding feature is truly necessary in the commercial arena, $1200 isn't a lot of money. Pretty nice!
Page 163 of the User Manual describes the 10 byte trigger setup. It's also a neat feature to be able to trigger on the end of a message (CR, LF, etc), the start bit or even parity error although, in my limited experience, parity errors don't happen. But I don't use long lines either.
For my purposes, I don't see any need to debug UART communications. I can just jam the stream into a terminal. But SPI and especially I2C decoding is much more problematic. Any scope that will decode these streams will be a blessing.
For I2C, it is possible to use a couple of logic gates to find the START condition and use the output to trigger the scope. Like this:
http://www.i2cchip.com/monitoring_i2c_bus.html This isn't a complete solution but it's a start.
SPI is triggered off of SEL' so no problems there.