General rules of thumb in my most humble opinion. If your bagseed has seeds, the grower either did not care or have enough knowledge to grow the plant to it's fullest potential. If it won't stay lit, it wasn't cured or dried properly. If it tastes "odd" it wasn't flushed before harvest. The basics of why we are here learning is that Light, airflow, nutrients, water and CO2, as well as timing make for a particular strain the possibility of reaching it's full potential. That list right there leaves a lot of thing to be fucked up, trust me on that. If a grower has the right lights, nutrients etc...., keeps the boy plants off the girl plants, flushes properly, harvests at an optimal time, drys slow enough, and cures properly, I flat GUARANTEE that the finished product will be be better than the original bagweed that had seeds. Just guessing that the bagweed with seeds was rushed to market and corners were cut. Those corners cut usually are cutting too early, drying too fast, and "what the fuck is curing?"
All that said, a specific plant can ONLY be as good as it's genetics allow, so if you get bagweed from Mexico, you know it is probably not as good as Vanilla Kush (2009 Cannibus Award Winner).