Here's an explanation from the filings of another company:
In the event of a "cashless" exercise, holders of the warrants can use the "in the money" value of the common stock into which the warrants can be exercised to pay for other warrants in an exchange of some of their warrants. For example, if the common stock of XXXX were valued at $1.00 per share, with a $0.50 exercise price, a warrant holder could exchange 100,000 warrants with an "in the money value of $100,000 for 100,000 fully-paid warrants, and so forth.
So it could end up being disadvantageous for the company...