Will emergency heat hurt the heat pump during defrosting

I've been told that a properly sized heat pump should run almost non-stop during the days that the temperatures are the normal low for the season and area. Any lower than the normal seasonal lows would kick the strips on.

The Lennox XP17 series is certainly capable of operating at lower outdoor temperatures than 35F with reasonable efficiency. The extended performance tables on these units are published by Lennox at the following url:

http://tech.lennoxintl.com/C03e7o14l/UfSSOXTazc/ehb_xp17_1512.pdf

Beginning on page 16, they list performance information for different indoor/outdoor combinations. For most combinations, the designed performance of the unit at an outdoor air temperature of 17F is about 60% of the full rated capacity.

One likely cause of the problem you describe is that your unit is equipped with an optional outdoor thermostat kit which is set much too conservatively (Lennox Part 56A87 or similar). This part is often used to automatically disable the outdoor unit during extremely cold temperatures and it is usually located somewhere in the control section of the outdoor condenser.