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.