At the end of the day, fashion is a business. And in order to stay in business, you have to turn a profit.
The couture collections have extremely high costs associated with them--high manpower hours, plus a heavy investment before each season in shows, etc. And while each piece is costly and it is certainly possible to turn a profit in couture alone, its very high risk because of the high costs that must be paid before each season and the relatively small and fickle customer base (it is a very small group of women who are regular consumers of fresh-off-the-runway couture clothing). Therefore, its wise for a design house to perhaps manage its risk by going into things like handbags, denim, perfume, ready-to-wear, and less expensive lines (Marc by Marc Jacobs, etc).
Also, as other have mentioned, these other products and lines can bring in huge profits and expand the company. They have a much wider customer base than couture clothing, too, especially handbags (even relatively expensive ones), and without volume, its very difficult BIG profits. Think of how many girls wear an outfit from Banana Republic or BCBG but carry a purse from Balenciaga or Chloe.
However, I do NOT think that going into these other products is neccessarily bad for innovation in fashion or design. You can think of it this way--the bigger the design company grows and the more profits and less risk it has, the more $$$ it can invest back into innovation and hiring the best designers and putting on terrific couture shows.
I'm cool with more people playing the game if they're going to add something to it, which I feel McQueen is. I may get shot for saying so, but I could live my whole life quite happily without ever seeing another Balenciaga motorcycle bag. When I find threads hanging off my person, or nearby, I clip them off, so be warned 
