This is not the only reason, but it is a very good reason. The response and accuracy of a 12 window strobe tuner is still invaluable. No other technology that I am aware is able to replicate the speed of a strobe tuner. Students and directors that learn to use it fix multiple tuning issues.
What I'm about to say is going to upset some people, but here goes: I don't enjoy spending a lot of time in band rehearsal or sectionals or lessons working on tuning. It's boring. This is why I don't like using a harmony director. I have been to all of the clinics and presentations at TBA, TMEA, etc. on how great they are. I have no doubt that they work, and I have no doubt that some of the best band programs in the state use them to great effect, especially with their higher level students. I'm also sure that it takes a considerable investment of rehearsal time to sit there and match pitches and make it so that students "get it".
A few years ago, I inherited a decent "middle class" HS band program with great potential in a mid-sized community. Because of my background in another successful large school program, I decided I needed to spend a big chunk of time in rehearsal "fixing" tone quality and tuning. This was pre-harmony director, so we did a lot of passing F around the room, unison drones, a bit of singing, etc. It worked - we made 1's across the board on stage and the recording turned out amazing. But guess what...we couldn't get a 1 in sightreading to save our life. I finally figured out by the 4th year that I had to spend more time in rehearsal working on this stuff or we would never make a sweepstakes. We didn't have a huge private lesson program to teach our kids these other fundamentals. Anything with reading skills, solo & ensemble, all-region, etc. we had to teach ourselves in rehearsals and the one or two hours of sectionals outside of the school day we had each week. We finally got a sweepstakes.
Back to harmony director. Use it I guess, but in our program, it's all about keeping the rehearsals interesting and alive, or we lose students. I understand that you have to work fundamentals and "pay your dues", so we still do listening exercises and daily fundamentals, but in a 50 minute rehearsal, we usually spend about 40 minutes on working the music. I have found that you can use a strobe tuner and do a quick tuning check of different sections on a rotating and/or "need to tune" basis, use a chorale for isolating/listening exercises, and do this on an average of 5 to ten minutes, work on a reading fundamental for about 5 minutes, and then hit the music. However, this is important: I also use the individual tuners in rehearsal. In my opinion, they are also a very important tool because they save time and make the students individually responsible.
We have had great success. I don't know if we sound as refined as some of the groups that use the harmony director, but we sound good enough to be in the hunt at area and state level competitions. We play challenging music for our classification, and I can say without hesitation that most of the students really enjoy band rehearsal. I have very few discipline issues in any level of our bands. It's not boring.
One last thing: the previous directors at my current school used harmony directors, so we have them. I will occasionally use them, but rarely. Ironically, they are the tool "gathering dust" in our band hall. The students sometimes turn them on and play little songs on them. They sound like the old Cascio keyboards from the 80's.