Well, just don't forget there's a potential drawback to that method — if you are on a call using a phone not connected to the OBi110 and someone else tries to initiate a call from another service (SP1, SP2, OBiTALK network, or even from the phone connected to the OBi110 Phone port by dialing **8 plus the number), the OBi110 will not detect the line in use condition and will blast out a series of touch tones in your ear and then connect that caller into your conversation. If you don't allow any other services to access the Line port and don't use **8 to place calls from the phone connected to the OBi110 (but instead always use # to get dial tone) it probably won't be an issue, but it's something to be aware of.
So, ultimately, an actual "barge in" feature (that doesn't require changing the "LineInUseVoltageThreshold" setting, and only works from the phone connected to the phone port, and requires a configuration option to be set that allows it) might not be such a bad idea. But in the meantime, for many users, changing that setting will work well enough.