Are fleas beneficial to anything?

Yes, fleas benefit the below nasty items:

As a vector:
Besides the problems posed by the creature itself, fleas can also act as a vector for disease. Fleas transmit not only a variety of viral, bacterial and rickettsial diseases to humans and other animals, but also protozoans and helminths.
Bacteria - murine or endemic typhus:[13]:124 Fleas have helped cause epidemics by transmitting diseases such as the bubonic plague between rodents and humans by carrying Yersinia pestis bacteria. Fleas can also transmit Rickettsia typhi, Rickettsia felis, and Bartonella henselae.
Virus: myxomatosis
Hhelminth: infestation of Hymenolepiasis tapeworm
Protozoa: Trypanosome protozoans such as those of the subgenus Herpetosoma, use a variety of flea species opportunistically as vectors.
https://en.wikipedia.org/wiki/Flea

More can be found on the negative effect and habits of fleas here.

http://extension.entm.purdue.edu/publichealth/insects/flea.html Of course, this negative is also a positive as they help to control certain populations growth and over-crowding through the transmission of disease.

In addition to the negative effects fleas are truly beneficial. The most logical way they help the ecosystem is as a food source for snakes, beetles, frogs, etc. These in turn become a natural food source for their natural predators, continuing up the line.

http://fleasbgone.org/fleas-natural-predator

They are essential in the assistance in the decay of food which in turn enriches the soil.

http://www.bihartimes.in/maneka/Minors.html