Because you are as retarded as he is. A build does not need to be permanent to be a build.
If it lacks any permanence, it isn't really a build, now is it? It sort of crosses the line into being a "loadout" if there is no permanence to it. When I slot autocannons onto my plane to convert it for hunting bombers, I'm not running a "new plane build", it's just a loadout. I didn't have to build a new plane.
If you can be anything at any time at cap there is no opportunity cost at all.
Well, that's not entirely true either. Just because you can be anything, it does not mean that you can be EVERYTHING. Take something like JA2. It's entirely possible to max every skill on every merc. Even if you do this out of boredom, you're not really going to actually use this capability. Just because you can do everything, it does not mean that you can do everything. Your mercs may be capable of functioning as a trainer, a doctor, a mechanic, and a combatant with maximal effectiveness. You can do anything. But you can't do everything, because that would require the ability to work all of these jobs at once. There's an opportunity cost: Every skill you can't directly employ and the cost of acquiring it is wasted. Odds are some lesser-qualified merc only capable of doing one job, perhaps not even at 100 effectiveness, is going to wind up handling tasks like hauling, fixing, and doctoring, while your super-mercs are working front line combat.
I believe a game I was really looking forward to, Shards Online, is going this max everything route. It holds zero appeal to me. It works out the same as a game with no builds because everyone is exactly the same. This is the exact opposite of what I think the core of an rpg is about.
For the reasons I mentioned above, "max everything" isn't necessarily WRONG. It isn't even necessarily unrealistic. Real people don't just run out of skillpoints and become forevermore unable to improve at anything, after all.
What do you think of the concept of the ability to max out everything, except there is also permanent death, which, in real life, is the balancing mechanism that stops people from doing that?