A while back, I bought a New Haven FL-9 and EP-5 from Mark Castigliano of New Haven Terminal/Branford Hobbies. The coupler pockets are held in by 2-56 hex head screws. Since nothing else on the model railroad uses hex head screws, and thus an additional tool is required for these models, I asked Mark about it.
Mark points out two benefits of hex head screws over traditional blade or Phillips screws:
1) You can put the hex head screw on the hex wrench and thus install it one-handed, handy if the other hand is busy, and
2) The hex wrench is less likely to mar a newly custom painted model than a screwdriver.