I don't know if you could call it flying for free as such, if I understand the OP correctly , he/she is flying as an "extra" pilot in a single pilot twin, therefore is not stealing anyone's job as there is no job to steal.
If he/she is, however, being allowed to fly the dead legs and log valuable twin time, then in todays climate it is not a bad idea.
On top of that, the extra practice at radio work, flight planning and general commercial ops is invaluable, plus more importantly, they are in the right environment to meet people and network.
It would certainly give a more interesting and relevant answer to the "how have you been keeping current" question at an interview.
regards
D-G