I've got several machines where I do use a vise. I don't use keys on the bottom of any of them. I have in the past, but my experience was unless I pulled the keys and machined them they didn't just position the vise accurately enough I didn't have to indicate it in anyway.
People in this group encouraged me to learn to indicate a vise, and that it wouldn't be that hard with some practice. "Don't be afraid to take the vise off. Every time you put it back on it will be easier." They were right. I still have some anxiety about placing a vise on the table and indicating it in. I've got two machines where I use a pair of vises dialed in together. That's a little more tedious but not horrible. A couple minutes with a brass hammer and an indicator at worst. If I am having a tough time it can take me five-ten minutes dial in two vises together. Usually not. If using soft jaws I'll still skim cut them so the average is consistent across the entire range.
Once in a while I slide a vise on the table, and indicate it so close I actually find myself moving it off alignment, because I can't believe it slid on so close the first try. Not all the time, but often enough that I no longer need to go on a social media group to share my excitement about it. Oh, I still get excited about it. I just don't need to share it every time it happens now.
I do smile when somebody else posts about it for their first time. I feel good for them. I don't make a big deal about it but I don't shit on their parade either. I am happy for them. The first time it happens it really is a big deal. Anytime anybody accomplishes something I try to be happy that they have improved their skills. Not sneer down my nose that I have been able to do that for years.
The one that really annoys me though is this guy. "Oh, I have keys on my vise and every time I put it on the table its accurate within 5 thousandths." That last five thousandths is usually the most work to dial in, and five thousandths over the length of the average vise is just not that good. He isn't doing anybody any favors in my opinion and his bragging about his short cut doesn't make sense to me. I think the keys are costing him time really. If he just eyeballed it on the table, put an indicator on it, and used the tap and travel method he could hit better than 5 thou just as quick. That's not even why I don't use keys though.
I feel like keys (on a vise) limit my ability to make maximum use of my machine or my vise. Basically I can only place the vise in 3 positions. Its going to vary by machine of course, but often I find if using the t-slots to position the vise I either can't use part of the work envelope of the machine or it clamps work pieces outside the work envelope. I don't care for the average cast mill vise with bolt holes either. Same limitations different cause. Yes I do have several, but none of them have keys on the bottom. I like what is marketed as "CNC vises" with a slot down each side and on the ends that you stick a clamp of some kind into. Sure, you either have to buy or make vise hold down clamps, but you can position your vise ideally for the work you want to do and make the most use of your machine and your vise. You can even clamp it down at almost any angle without the need of a rotary base and without the loss/change of horizontal accuracy that might come with an inexpensive vise and rotary base.
Now the guy who is proud of his keys. I try not to shit on his parade either. Its entirely possible that whatever level of quick accuracy his keys provide is good enough for the work he does, and ultimately that is what matters. If you can produce parts within spec everything else is wasted motion.
Now, I don't own any really expensive vises. Maybe the keys on the bottom of a Kurt or Orange vise are dead nuts and maybe they only get used on machines whose t-slots are dead nuts.
I do use keys. Just not on vises. I use a lot of shop made fixture plates for cutting rectangular bar stock larger than I can easily hold and position in a vise, and for holding more pieces of stock than I could hold in the vises I could fit on the machine table. If I ever perfect my own ideas for a universal fixture plate I may never put a vise directly on the table of several of my machines again, but that is a whole different topic.