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The XTAL Trim is basically there to compenstate to the real world inaccuracies invlolved with the Crystal and PCB which effect the overall input frequency to the chipset.
As the device uses a low cost XTAL instead of a TCXO, it becomes important to carefully calibrate the the RF module.
The spec allows for certain margin in the spectral freqencies. In BLE this tolerance band is around 125kHz. The XTAL trimming process allows you to tune the device to operate as centrally as possible to this margin.
Effects of incorrect tuning will be seen on case by case basis as they depend on external factors and componets. It may be possible that the tuning value is 0, and hence there might be no actual issue observed. However, in most cases, the end user will see a reduced range and high packet loss (leading to low throughput) for an incorrectly tuned module.