I personally think looking for a rescue with a proper temperament is less time consuming then finding a breeder breeding what you want!!
As for weight pulling being a sport...IMO unless your dog is pulling a loaded cart around a farm, completing chores, or making money doing it, it's sport...I mean lots of things humans do for sport or fun is hard work mentally or physically...football, chess, boating, etc. I know many rescue dogs who would weight pull or hunt, no problem.
I'd rather rescue because if open my local craigslist or shelter's page I see 15 + dogs in need that would be a good fit for me. When I need a dog to work I plan on doing the same...yes it will take a bit more time but no more time than looking for a breeder. I may in a very long time get a dog from a breeder, but probably only one or two dogs in a houseful...
Out of curiosity, to the OP: do you ever think you will have just a pet dog in the future? then will you rescue or would you still get from a breeder? Also, let's say you were looking for a pup from a breeder and came across a two year old dog in a need of a home, say it WAS a rescue pup but now needs to be rehomed, it never worked but without a doubt has the correct temperament...would you not take the dog because you didn't know the lineage?