Would the updates be very useful for someone who can read a little Hebrew/Greek, but is not a particularly skilled Greek/Hebrew student?
IMHO, not especially. If you were keen to get into critical studies, then I think that would be the time to look at that particular purchase. If you have a paper GNT like the UBS4 or the NA, if you find yourself looking at the alternative readings at all regularly then it seems like you should do it.
If you are more trying to work out what the Greek i(or Hebrew) s saying, then stay with this GNT or GNT-28 (HMT-W4) module. YOu may look at adding the syntax module, that is quite interesting and helpful. It is not that expensive to add the BHS-T or the NAS28/UBS4-T later on if you move more that way,.