What is the acceptable range for bandwidth utilization factor in OFDM systems?
It depends how good your system's anti-aliasing filter is. Your transmitter/receiver will have an antialiasing filter after the DAC (transmitter) or before the ADC (receiver). This filter will be designed to pass the band of frequencies that contain the signal of interest, while cutting off any content at higher frequencies that would alias into your signal's passband. 

The higher the bandwidth utilization factor, the closer the passband edge is to the Nyquist rate, and thus the narrower your filter's transition region. A narrower transition region requires a more complicated filter (i.e. it will have a higher order, which can require more discrete components). Since such a filter would be implemented in the analog domain, this causes an increase in:


Design complexity
Physical size
Cost


OFDM is often used in widely-adopted wireless standards. In these cases, the final bullet above is a prime concern. Take WiFi as an example. WiFi transceivers are made by the zillions; even a small cost savings that can be realized by using simpler analog filtering can add up to significant money when multiplied by the numbers of units that mainstream electronic devices are sold in nowadays.