Though it had been rumored for months, a new report indicates that Darth Vader will, indeed, play a part in the still untitled Han Solo film. Seeing as the movie is fairly late in its production, with new director Ron Howard taking over from deposed co-helmers Phil Lord and Chris Miller, it's fairly unlikely the villain's role will be all that substantial. However, no matter how large or small it ends up being, the Dark Lord of the Sith's presence may hinder Han Solo more than help it, for a very simple reason: Involving the character undermines the stated purpose of the anthology movies, which is to explore unknown aspects of the fan-favorite galaxy, far, far away.
As an iconic character who has consistently moved Star Wars merchandise since the 1970s, it's clear why Lucasfilm and Disney would feel the need to put Vader into as many films as possible. His likeness sells toys and products, and a renewed interest in the character will never hurt the company's bottom-line. It's also an act of fan-service, designed to please the audience without necessarily forwarding or bettering the story.
In addition to merchandising, seeing a familiar character among a cast of unknowns helps viewers situate what they're watching in the universe from which it comes. Vader is a practical choice for a well-worn character to incorporate into a new film because his iconicness doesn't depend on the actor physically playing the role, or require awkward CGI reconstructions; it relies on the familiar black armor.
However, in the case of the Han Solo film, the title character should be the one that connects the story to the larger world. Solo is definitely not an unknown character; he's appeared in the original trilogy, as well as other media, not to mention the first chapter of the latest trilogy. However, Disney did away with much of the franchise's expanded universe in 2012, so even longtime fans know little about his early years; thus, compared to Darth Vader, they know very little about Han Solo.
Even more important are lesser explored characters audiences already know, like Lando Calrissian, and those who had a major influence on Solo that have never appeared on-screen before. The Han Solo movie is just as much about them as its title character, and bringing Vader back into the fold just pulls focus from Solo and those important to his story.
Nearly every Star Wars movie already features Vader prominently, and the character also has several excellent comics that explore his life both during the original trilogy and in the gap between Revenge of the Sith and A New Hope. Because of his omnipresence, it's unlikely that a short appearance in an anthology movie will yield any significant revelation about Vader's character or history, further limiting his value to the film. The Sith Lord's own outings give fans a deeper, fuller picture of him, and the purpose of the anthology movies is to provide some of that depth for those who haven't been explored in the same way in present continuity.