There are a small number of successful Cocoa apps written in bridged languages, so you don't necessarily need to use it that much, but you do need to learn it. People who try to learn to write Mac OS programs in bridged languages do themselves a huge disservice. The bridged languages are great tools, but they are the sort of things that allow someone with a knowledge of the bridged language and Objective C to become extra productive, not skip learning Objective C.
In order to use something like PyObjC or MacRuby effectively you need to really have a good understanding of how the native runtime works to deal with all the impedance mismatches that can occur between the bridged language and the Objective C runtime.
