A prostitute is defined as someone who exchanges sex for money. To the extent that the term has a negative connotation, it is because the act itself of offering sex for money is usually seen in a negative light. I occasionally will use the colloquial term "hooker" although that term does imply more than just the business transaction and unfairly blames the woman who is offering something that men are willing to go to the street and pay for. I avoid the term "whore" since it takes that implication to another level, though I may occasionally refer to a person as a whore if I think it's an appropriate descriptor (ie, someone who offers up their dignity on the cheap- see "politician").
The term "sex worker" seems to be an attempt to legitimize the profession, if you will. If we believe that prostitution is damaging and harmful to the people who are involved in it (frequently not of their own will), then legitimizing it with a less offensive description would seem to be harmful to them as well. I would not refer to enslaved or compromised people as "workers" in any other context, after all. I believe that there are contexts in which the term sex worker is appropriate, but using it to sanitize prostitution doesn't seem like one of them.