Hi,
The behaviour of the second inlet of line~, and of the second and third inlet of vline~, are not documented in the help patches. Only the "all parameters in one message to the left inlet" way of working is described in the help patches. The behaviour of the other two inlets is not entirely obvious: one can easily guess that they are used to set the ramp time and (for vline) the delay; however the fact they "forget" the value after the first message sent to the hot inlet is in contrast with the behaviour of most built-in objects and hence is not totally obvious.
I wonder whether they are deliberately left undocumented to "discourage" their use, with the idea of removing them in the future...?