I do agree the leash can send off wrong signals when dogs are being introduced on leashed but in the world where car roams, and other danger lurks, leash is always the best. When I see other Yorkies in my area, there are a few here, I do have the tendency to let them meet on leash and I never allow him to do it with other bigger dogs (let the leash loose no tugging on the leash) and usually they are okay and if they aren't, we discpline them and start afesh and it usually works but then I always like to invite them to my house and we let them off leash to play
. They tend to try to settle on a pack ranking the first time they meet but after that, they are okay. You just have to supervise and let them learn how to socialize unless the other dog is so aggressive as to attack non-stop then it should stop. And I think Yorkies know when they see other Yorkies...they know they are of the same kind and they tend to do better.
We have a Yorkie gang in our neighbourhood-4 of them! And I would just love to have them all over to my house and play!