Well, you'd want both. You want someone dedicated to that role of business analyst, and then you also want your senior engineers to be able to do it, but just not focus their time on it. The understanding is key to improving their execution (unless you think BAs will provide you a 100% perfect and complete spec?) and helps with the turnaround time, so that not literally every ask to the business owner has to go through your BA. And maybe even most of the questions you'll be able to juts answer on your own, and then the turnaround time is literally zero.