I've posted before about my Butterfly brain so this month's Blogging carnival at JMTC is really appropriate for me. I'm not sure there's a definitive answer but perhaps my own experiences can help you analyse your own playstyle/experience so you can maximise your gold/time system.
For me, diversify is my natural habit - I have most profs maxed now (except BS & Eng) & although I am currently spending most time on JC & Inscription, I'm also popping over to my LW'er when a blog post comes up that reminds me of something or to my bank alt mage to do a quick teleport around the capital cities for vendor items.
I've tried to streamline my routine to give myself more time levelling or learning other markets & to be honest, I'm not sure I like being organised like that! Am I missing opportunities? Undoubtedly. Am I having fun anyway? Most definitely! So I guess the question of whether to specialise or diversify is very much a personal choice, made simply by trial & error & limited only by the number of maxed profs/various alts you have & the amount of play time you are willing to spend on making gold.
The bulk of my gold comes from glyphs at the moment - using TSM (Trade Skill Master) has speeded up the whole process so I now cancel/repost several times a day which has made a huge difference to my income.
I also do a lot of ore prospecting for my JC/Enchanting alts - I sell uncut & cut gems, rare quality (blues) necklaces & rings from Alicite & hessonite, disenchant the 'greens' for dusts/essences & craft scrolls.
I dabble in Mysterious Fortune Cards, I watch the markets for Darkmoon Faire Cards (& buy when I see nice prices!), I occasionally craft Relics, I make PVP leather gear, leg armours & mammoth mining bags. I've just levelled tailoring so I've added netherweave bags to the mix.
So you can see, I have a lot of fingers in a lot of pies BUT I'm not an expert in any of these markets - I don't dominate any of these markets & I'm not trying to. That road leads to stress for me & that's not why I play this game. I'm more of a 'go with the flow' type person so a bit here & a bit there suits me.
Is this attitude/personality thing ringing a bell with you? Then don't worry about specialising, work with what you've got & make the best of it. If you happen to stumble into a good deal or a quiet, untapped corner of the market, then you'll have the skills to make the most of it. You're reading gold blogs if you've found this so you're ahead of many other players anyway.
Is this attitude/personality thing sending chills down your spine or making your toes curl in horror? Well, I guess you're a more organised person, one who, perhaps, is more suited to specialising in one or two markets & doing them really well? You know what? That's good too! There's room for all of us in this game after all :)