Well, you don’t NEED 140/150mm, but why not have it if you can so long as you’re not compromising your main goal. I don’t know why so many people are worried about being overbiked when in other areas of their life they will be over something else – like over car’d, over housed, over PC’d, over lawn-mowered, over home cinema system’d etc, etc.
It will give you options and the ability to do something you were not originally intending to do when you first purchased your bike should you fancy it one day. However, there are many great 140/150mm All-Mountain bikes out there that will climb very well, though they will all be heavier than a shorter travel cross-country orientated bike which may or may not turn you off them as an option, but then again I think people put far too much emphasis on weight when selecting a mountain bike – 5lb (isn) separates the lightest and the heaviest (within reason), which is a small overall percentage of your all-up weight, so ultimately not an issue.
Ultimately at the price range you’re looking at (and a bit below) there really is not a wrong choice once you’ve decided on the style/type of bike you’re going for.