Should fashion designers start furniture lines?  

I think that...
Fashion designers should do just that,and not diversify into other areas of the design world ie. perfume/furniture etc. but am i wrong? Furniture designers have gone over into the lifestyle area havent they? some are a little too prolific in my own opinion, Arad/Starck to name two.Maybe designers should be encouraged and applauded in doing what they do ..Design?. In short i dont know that there is a short answer to your question.

I Think........
Theoretically, a good designer should be able to turn their hand to most disciplines.
That said, I can't actually think of any fashion designers that are doing furniture lines?

Here are two
Armani and Calvin Klein- to name but two!

Companies of that size
Companies of that size employ sometimes hundreds of designers, so probably hired furniture designers to design the ranges.
I don't see a problem with that, it gives talented designers a job
I don't imagine calvin Klein has actually designed anything that has his name on for a good many years......
With designers like Giorgio Armani, they have a strong house style (and i stress the word 'style'..not fashion, and it is this that people will be buying into with the furniture range.
I dare say he oversees the designing, but would hire the best furniture designers he could get. Surely no-one is that egotistical that they would do otherwise ?

The question should not be should they branch out, but rather...
can they do it well?
Anyone with alot of brand recognition can fan out into design, or at least branding of other products.
Should Martha Stewart, who is a homemaking "designer", be pedalling all the !@#$% she did through KMart.
Should Michael Graves morph back and forth between architect and designer, and occassionally into Michael Graves utensil and appliance designer?
Should Eddie Bauer, which designs mail order and retail business models, become the "designer" of Ford SUV interiors?
Should baseball great Ted Williams, who designed nothing but a strike zone (though he did that the best of anyone ever!) have "designed" endorsements of sporting goods for Sears?
The common thread here seems to be that people will do most anything they can get paid enough for. Why begrudge them that?
Most of these cross designers probably hire experienced designers to do the actual design work anyway, or else just shape the plastic around the machine themselves.
I'm just glad people are paying more attention to design, regardless of who's name it is done under.