there is only one reason to do a trade for portfolio shoot, and that is if the model has a particular look that the photographer wants in their portfolio. in blunt terms, if she's hot enough to make you look good by extension.
i'd really like to see the business case for spending the time needed to shoot and process one to two of these per week. it should be easy to track the value of the freebies at your hourly rate against the revenue brought in as a direct result.
we all have to do some promo work from time to time, but giving shit away or working at cut rates is what devalues the industry. i don't see many of the girls in this world doing that.