She's right, most of them won't serve well in that roll, but it's not because modern phones are poorly designed. Instead, the role of the phone in home defence has changed.
"What? Changed you say?"
Well, yes. We have advanced beyond the days of the solid American phone-club. Today's phones are mostly manufactured by the Japanese and are designed to serve as disposable tools for distracting an opponent, in a manner similar to the way "ninja stars" have been historically employed.
The new tactical phone home defence doctrine goes something like this:
1) Hear an intruder.
2) Dial 911.
3) Put the 911 operator on speaker phone.
4) Throw the phone at the intruder.
The intruder will be struck by the phone and distracted from attacking you. He will leave, clutching his new phone and trying to explain politely to the 911 operator what is going on. If he does not leave, the user is instructed to ask his girlfriend, spouse, room mate, or minor child for their cell phone and repeat steps 2-4. For especially persistent intruders it may be necessary to use your phone from last year or the year before. Eventually, the intruder will become frustrated and depart. At that point, the phone's camera and microphone can be secretly activated to identity and apprehend the intruder.
Of course, no tactical doctrine is perfect. Americans have found it necessary to add another step:
5) Shoot the intruder.
Most reports indicate that this seems to work even if steps 1-4 failed utterly. In fact, some people argue that steps 1-4 are entirely unnecessary. However, most experts recommend maintaining the five-step process in order to provide something for liberals who don't own guns to do in an emergency.