I knew I forgot a word in my last post: It'd also be repetitive. The same thing happening over and over gets boring. Oda hasn't had a problem breaking formulas before. For example, Luffy took on the second strongest enemy in Punk Hazard rather than the strongest, which went to Law and Smoker. Sanji also tangoed with the strongest, while Zoro took on the weakest main enemy. There's also all of the Strawhats taking down Oars. Jr, and Zoro fighting Hody Jones, the main villain, in Fishman Island. Sanji is currently the one facing off a Yonko's crew, rather than facing the third strongest. It's arguable as to who is stronger, Fujitora or Doffy(most likely Fuji), but not Luffy nor Zoro nor Sanji fought him, but Sabo. It's good for Oda to do new things rather than conforming himself to the same predictable things all of the time.
What's so bad about Luffy and Law teaming up? You get to see your all-out Luffy. Law fans get to see Law do something cool and live up to his development with Doffy rather than Oda squandering him. Oda gets to dish out some pretty good writing, better than the "Luffy alone" alternative. Oda gets to break repetition, and do new things. We get to see what the badass villains of the New World will be capable of through Doflamingo. What cons do you see that you just need for Luffy to do it alone?