Michael,

yes, the SCL line can be hold down, but the intention was not to do many other things, only to synchronize.
You must use interrupts to handle the I2C transfer.

So this is not an I2C property. Also on all other protocols you can lose speed, if you not handle any transfer immediately.
The advantage of I2c in comparison to using the UART: the transfer was only delayed, but on the UART the byte was lost.


Peter