That's the same thought that I have heard from a lot of people. Unfortunately, it doesn't work that way for most of them.
A trap gun normally shoots high when you're looking right down the rib. If you just raise the comb on a flat-shooting gun to make it shoot more like a trap gun, you will be seeing a whole lot of rib. If you're used to it, no problem. But if you're going back and forth, it's going to be disconcerting.
Secondly, that's a whole lot of messing around with the comb every time you want to shoot a different game. Let's say you're going to shoot a "club championship" - a couple of rounds of trap, some skeet, some sporting or 5-stand. Maybe you shoot skeet first, then trap, then sporting. So you'll have to adjust the comb going from skeet to trap, then adjust it back. How are you sure you got the comb in the right place? If you miss, are you going to have doubts and start getting out the allen wrenches to mess with the comb again? Most adjustable comb hardware that comes on factory guns isn't designed to be adjusted over and over and over again. Some of it can slip over time.
I dunno. If you're primary game is trap I think it's harder to have a "do-all" gun, unless you like a flat-shooting gun, or are willing to put up with swapping barrels and stocks (some guys do this with Perazzis, Krieghoffs, or Berettas). If you're primary game is skeet or sporting, I think it's easier to have a "do-all" gun.
All just my opinion though. Others will certainly differ. If I were doing mostly trap with some skeet or sporting, I'd probably get a 682 trap combo (available used for about $1600) and send it to Rich Cole for a set of 30 or 32" sporting barrels and a sporting stock. The barrels should run about $1000, the stock somewhere between $150 and $900. You'd have a "do-all" gun that had no compromises for under $3000. Add subgauge tubes and you'd be set for any clay target game you ever wanted to shoot.