Its not because square waves put spikes on input capacitors and other problems. PFC circuits and power supplies are damaged by square waves.  Pure sine inverters and ups units are also more efficient than square wave inverters. Official Dell documents say Power Protection & Back-upIn the case of a power outage, be prepared with an Uninterruptible Power Supply (UPS) that can provide business continuity to prevent critical data loss. For Dell  desktops and workstations that utilize Active Power Factor Correction (APFC) power supply units, Dell recommends Universal Power Supplies (UPS) based on Sine Wave output, not an approximation of a Sine Wave, Square wave, or quasi-square wave (see UPS technical specifications).  
I currently have a Simulated Sine Wave UPS (1200VA, 720W), is it safe for me to use this for my Alienware Aurora R10 which has the 1000W PSU? 
Aurora R10, Simulated Sine Wave UPS vs Pure Sine Wave UPS