According to Shmuel there is an extra pasuk to teach that the issur of meat and milk is binding even on top of a preexisting issur of neveila, therefore one who eats neveila cooked in milk would receive lashes. However, the gemara cites an argument between R. Ami and R. Asi limiting the discussion to be whether one who cooks neveila with milk would receive lashes for the act of cooking, but all agree that since the eating is already forbidden due to the issur of neveila, the prohibition to eat meat and milk isn't binding on top of the preexisting prohibition of neveila.
Based on this, the Rambam (machalos assuros 9) paskens that one who cooks neveila with milk would receive lashes for cooking meat and milk, but one who eats it wouldn't receive lashes for eating meat and milk. However, both the gemara and the Rambam (in mishne torah) are silent regarding the prohibition to benefit from meat and milk. Would that be similar to eating and not apply to neveila, or would it be compared to cooking and apply to neveila?
The Dagul M'rvava (y.d. 87) cited by the Pischei Teshuva (6) quotes the Rambam in his commentary to the mishnayos on krisus who suggests that benefit and eating go hand in hand. Since the issur to eat meat and milk isn't binding on top of the prohibition of neveila, the prohibition to benefit is also not binding. The Pischei Teshuva quotes in the name of the kanfei yona who disagrees with the dagul m'ravava, and the Chasam Sofer who says that one who relies on the dagul m'rvava can't be rejected, but in his opinion the kanfei yona is more correct.
The source of the dagul m'rvava is the Rambam in what he refers to as a נקודה נפלאה in his commentary on krisus. The Rambam asks that the issur to eat basar b'chalav should be binding on the issur of neveila since it is an איסור מוסיף by virtue of the fact that meat and milk is forbidden even to derive benefit from. To this the Rambam says that the issur to eat and the issur to benefit aren't independent entities, rather the prohibition to benefit is an extension of the issur to eat (we see a similar concept in succah 35a by an esrog that is forbidden to eat where rashi writes that it doesn't qualify as לכם. Rashi explains that "lachem" implies that you can benefit from it in all ways - an esrog that is forbidden to eat is missing a major source of benefit). Based on this Rambam, the issur to benefit isn't considered a new issur to cause the issur achila to be binding, and the dagul m'rvava therefore proves from here that even the issur to benefit itself wouldn't be binding. There is a lot of discussion about this Rambam because he seems to say that the approach only works because there isn't an independent source to forbid benefiting from meat and milk, yet in sefer hamitzvos he writes himself that the third repetition of לא תבשל גדי בחלב אמו is an independent source to forbid benefit.