For me there are a couple of reasons to use a DJH or other RTR (Ready To Install?) motor and box combination is:
1) The kit manufacturer recommends it - because the kit is either designed to fit, or it has been tested and confirmed to be a good choice.
2) If you are paying someone whatever the going rate is for building a kit for you, it may well work out cost effective to use instead of paying for a box to be built, tested and installed.
However, the cost and flexibility of the HL range is great for my needs. I did try using a Markits box for a 7mm loco but just could not get it to run quietly (no doubt user error).