Hi. I have liner over the stairs. For sure do it. It makes it looks great. My only concern was slippery steps. I was in a pool where the liner over steps were slippery. My PB assured me it was a water condition created issue. He was right. If you keep your pool water balanced there is no issues of slipperiness and no need to worry that it will rip easier than the rest of the pool. I wonder why he does not recommend it? I feel it gives it such a polished look. Add a great coping and you wouldn't know it's a liner pool. Maybe if you have a dog that would be in and out it's a concern??
Stairs are tricky to measure and your builder may feel unsure of himself. If not done right you will have wrinkles or heavily stretched vinly. If it's not must have and your builder doesn't want to to do it then take the pass. If it's a must have then make it happen.
Our previous pool had vinyl stairs. There was also some type of padding under the vinyl which made it a cushy seating area also. No problems at all. If i absolutely had to pic nits, since the first step was covered by only inches of water, it became just a tiny bit more sun-faded over time than the rest of the pool....though i'm sure I was the only one that ever noticed.
We went vinyl over stairs as in my opinion it gives a more finished look than the standard plastic or whatever it is they normally use. If you decide to do it I would recommend going with the cement base instead of steel as the steel feels really springy when stepping on them.