I think both of you are right. Specialization is the key here. You can make boatloads of cash doing just the gathering missions and selling mats, but the same applies for just doing the crafting (while obtaining the mats from the GTN). So there's a definite market for each strategy.
What is really suboptimal is if you try to do both. E.g. when crafting, you can queue up all your companions, churning out items at a steady pace, with hardly any intervention. I can generate a boatload of items per hour that way, while I use my character to do the fun stuff. If I were to gather my own items (especially the more exotic ones) that means my companions are NOT generating items (= loss of income). And there's NO guarantee I'll actually get the exact amount AND type of mats I need for specific items (not very efficient). Or I have to go out to get mats myself, which I find about as interesting as watching paint dry. Better to scourge the GTN looking for bargains (large stacks usually can be acquired at a decent price per item).
Same way the other way around: you can make good money just gathering and selling the mats (mostly for people like me), and don't craft anything at all (better have extra gathering capacity). The efficiency argument I used earlier does not apply in this case, since you are not crafting yourself. Just put any crafted materials on the GTN and people like me will buy them. And again, let your companions do the heavy lifting, and use your character to do the fun stuff (unless you enjoy gathering of course, to each his own