Being stuck driving a Porsche isn't a bad thing, but variety is the spice of life. It's even more important in a world where not everything is an iPhone. If a Windows or Linux shop recruits you, you can forget all of those extra Objective-C extensions you learned because they'll be of no use.
Portability is valuable. Apple (and iPhone users) are lucky that Apple's products are so good -- they're a powerful carrot to get someone to rewrite their code in Objective-C, which isn’t terribly efficient for developers hoping to maximize their efforts across the myriad devices available to consumers.
(Note: With clever use of gcc, Cygwin, and GNUStep, you can use Objective-C elsewhere, but this is only for the strong of heart.)