[thelist] [OT] Is fixing on a certain platform/programming language good or bad?

[VOLKAN ÖZÇELİK]

Hi an off-topic question that has been pondering in my mind from the
time I began my professional life. I am sure you have had (and you may
still have) similar thoughts before:

I am more of a web application developer, and a bit of web designer.

My question is,

is focusing on a certain platform and programming language (say J2EE /
Java ) and learning every bit and piece of it, mastering in it and
eventually becoming a guru of that language/platform

or 

swapping between several platforms (say Java / J2EE and C# / .Net) better?

That is to say shall one be "the best" of a certain thing. Or shall he
know bits and pieces of several things at the same time? Of course
s/he should focus on a certain field (it is impossible master in Web
Programming / Design and MFC Component Development  concurrently).

What I ask is "is expanding the spectrum  (learn c#, learn php, learn
jsp/j2ee, learn coldfusion) good or bad?"

Here is my logic: 
Well for the time being, I code J2EE in proffessional life, and most
of the challenges I face are in conceptual level. That is to say, they
can be either implemented in framework X or in framework Y. So
designing the pseudo-logic is the -let us say the "engineering" of it-
is the hardest part. Implementation is not a big issue.

But the facts of life are somewhat different: 
If I resign my job (or say my boss kicks my ass) and look for another job:
Saying 
"Hey I've coded in J2EE environment for several years and it's true
that I do not know any single bit of c#. But I've mastered a lot of
things at conceptual level. I will learn faster than an average
individual."

will lose while

"Hey, I know J2EE best, but I code C# at home as a hobby. I know most
tips and tricks of .net platform at least as well as an average
employee of yours know. It will take some time as per the transition
period, but it's not a big issue for me. I'll be fully efficient
within X weeks."

is a winning one. 

Hope I'm clear on expressing my thoughts.

Any opinions?

cheers,
Volkan.