Usually it is due to licencing terms. But still it may be crappy :-)

Some parts are closed, the glue between kernel and proprietary parts is open (mainly due to GPL requirements), see some details here http://garage.maemo.org/pipermail/cx...er/thread.html

As for cx3110x (the glue) - its quality is not exactly stellar, 770 version being much worse with CPU. For N800 version CPU consumption it the open part should be fine/better but it is hard to say without benchmarks. The wi-fi chip is connected over SPI bus, OMAP2 has SPI interface which handles SPI communication without needing much CPU, 770 uses McBSP chip (serial port) for SPI and busy-loops waiting for each result from the chip which eats a lot of CPU.