If the outlook is short-term than I agree. You will always be able to hack out a quick solution in PHP/Ruby/Lisp much faster than Haskell. But this advantage is only unmitigated if you fail and the code goes away before it has to be maintained. If the code is going to be around for a while it's going to need to be maintained, and here the advantage of Haskell grows with the code base size. It's a bit deceiving because the type of bugs that Haskell will catch automatically are not so much of a burden in a greenfield Ruby project, rather it's years later after bitrot and layers of iteration and no one really understands the whole system anymore that the advantages of Haskell really shine.
Granted, if you don't know Haskell then it's not worth the cost to learn with everything else you need to juggle when doing a startup, but if you can, I believe there are significant long-term advantages.
It really depends on the type of risk. My assertion is that most startups have most of their risk in the market bucket rather than the technical bucket. If you can move some risk from the market bucket to the technical bucket, and compensate by using Haskell, that may be a good tradeoff.
in the case where your risk is in the market bucket, you may go through many just-barely-working prototypes before you hit product-market fit. at that point, if you've saved some time using Ruby (say) because of external factors like Heroku, Bundler, and ruby-friendly services, you're ahead of the game, even if your tenth prototype sucks and needs to be rewritten.
Granted, if you don't know Haskell then it's not worth the cost to learn with everything else you need to juggle when doing a startup, but if you can, I believe there are significant long-term advantages.