Can't see any good reason why not. It may also help with youth development as the likelihood of young players getting a run out increases.
I guess there's the argument that a team could make six subs in the last 10 minutes to kill time but that can be sorted out by just allowing substitutions to happen without all the rigmarole of holding boards up. The fourth official can keep an eye on the number of players on the field and be responsible for the substitutions without the need for the ref to get involved in them.