UrRong! It could very well be that Brown is a highly respected partner in the firm without a hint of drama in their ranks. However, it's an undeniable fact that the Smiths have a familial relationship, as evidenced by the fact that their two surnames are combined in the name of the firm. Whether Brown is respected or not, their name placement at the end of the firm's title does still point to their status as the third wheel. To see it any other way would be optimistic at best.
To further this point, let us refer to the originator of this idea: Nikhil. He writes that when he sees firm names like Smith, Smith & Brown, Attorneys at Law, they make him wonder how Brown feels about being the law firm’s third wheel. And I have to agree, because it's hard to deny that that's exactly what they are: the third wheel. It's not to say that they're unimportant, but their name placement definitely gives the impression that they are the addition to an otherwise complete family of Smiths.
So while it may be possible that Brown is highly respected and is not viewed as the third wheel in any capacity, there is still a certain truth to Nikhil's assessment that Brown may feel like one. UrRong if someone says otherwise - after all, you can't deny the power of implication inherent in a firm name like Smith, Smith & Brown, Attorneys at Law!