EUR 600 can be invested in stock which will grow in value over time and permit one to retire a very wealthy person if one is smart enough.
But, that is not the point.
For those who travel, know how to read the terms & conditions of a voucher (or other document) and understand their limitations, the variance represents very real value.
Whether the difference between the voucher and cash amount represents value to any given individual is a matter of personal travel habits and preference.
Rather than advising others to do or not do what I would do, I would advise others to consider the costs & benefits.