I thought about those shortcomings of the system related to power cuts, or phone line cuts, etc.
But what it comes down to is, if a robber is determined enough to cut those, then chances are, no alarm system that I could afford would be able to stop them. So I just won't worry about it.
Plus, if I had that kind of valuable stuff at my house, or if I lived in a fancy enough house that attracted those kind of robbers, I probably would not trust X10 with my security. In my area, it's mostly petty theft, smash the window, grab and run kind of break-ins for both cars and houses.
My alarm is mostly for:
1) Letting me know when there is a fire
2) Letting me know when someone attempts to break in (broken glass, open door, etc.)
3) Shocking the robber who tries to break in through visual and auditory channels and hopefully force them to run
To be honest, just having an ADT banner outside the door does a LOT to discourage the potential robber and have them break in to another house that does not have a security system.
My car just got broken into a couple of weeks ago. They smashed the window, went through the middle console, did not find anything worth taking (even though there was a brand new GPS receiver) and moved onto the next car on the street. They hit 4 cars total. Took nothing. I found out that my factory car alarm only goes off when a door is opened. If you break the glass and lean in, nothing happens. They probably knew that because all four cars they hit had the same type of factory alarm so we had no idea what happened until the next morning.