He's against it because it's not repair.
When you repair something properly, there should be an expected reliability period, which you perceive as "repair warranty" on a piece of paper at the end.
Even if it's non-professional self-repair, it should still follow the basic human logic. I bet you don't patch your leaking sewage pipes with dried cow shit and bubble gum, even though it might work in 30% of the leaking pipe cases. If a passenger door in your car won't close, you don't use a rubber-band-to-the-driver's-door hack as a permanent solution.
With "baking", results are unpredictable at best. You may either throw your card into remission for a few days, weeks or months, or you can immediately kill it.
In late 2000s, when this "baking" craze has took off with great speed(due to 8000/9000-series cards dying like flies), it bred a shitton of "PC repair specialists", who's only real skill was to be agile and endurable enough to circle-jerk a hot air wand for 10 minutes. "Baking" and "reballing" became a universal remedy for any GPU/Motherboard/Laptop problem, regardless of the cause: first you bake it, and if it won't work - reball it.
I make the majority of my living with component level repair, and I can tell you an awful lot of ridiculous stories about baking.
About once or twice every month I'm getting either an ASUS Zenbook or any other ultrabook w/ most of its components soldered on motherboard, that was previously "repaired" or "unsuccessfully repaired" in various small workshops in Kiev. Some of those have faulty soldered RAM or VRAM, some of those had trivial backlight issues, but what was the first thing those "repairsmen" did? Of course bake the fucking GPU or the PCH! Any AMD-based HP envy laptop comes pre-filled with solder flux and remnants of aluminium sticky tape. Some managed to charge customers upwards of $150 for a GPU "replacement", even though the original GPU corner standoffs were still intact.
Best case scenario - the chip stays alive and all I have to do is clean the shit out of it beforehand, and then eliminate the original problem and do some thorough testing. Worst case - they mercilessly kill a perfectly functional GPU for no reason and the customer still has to pay me for diagnostics.
Just last month I had a Dell Precision M4600 which only needed a power jack replacement, but some dumbass thought that random shutdowns were caused by a malfunctioning NV Quadro GPU, so he baked the shit out of it. Now it's nearly dead (some VRAM banks died, and GPU was giving artifacts even after VRAM fix), so after the trivial jack repair I had to remove the GPU, leaving the customer only with Intel HD graphics (though, he was very happy with improved battery life).
I also have an entire collection of GPUs which were tortured by baking, while the real problem of "no image" or "artifacts" was totally unrelated to the GPU.
So, as a professional I don't give a flying fuck if baking helped some people to prolong their GPU life. All I care about, is that sooner or later this GPU or laptop is gonna end up on my desk and I'm gonna have to deal with consequences of someone else's incompetence, starting with explaining the concept of "reflow and reballing is not repair" all the way to breaking down why do I charge so much comparing to those other guys (since I actually have to buy replacement parts and include them in my final bill).
