We considered it a while back and it may happen but more mods doesn't necessarily equate to less spam and there are a few active mods/admins who do a lot to remove spam - JeffDM, lundy, Mr.H, Kasper, audiopollution, me and a couple of others. Some may not post a lot but they are active. As you said, this site is quite popular and from one of the info pages gets over 1 million unique views per month. Over the past year or two, spam got pretty crazy so the only real solution was to automate the spam catching and that's been implemented.
It is based on a filter that needs to be refined but some days, it catches as many as 45 unique threads in under 24 hours. To clean that up manually is not pleasant and we've had to do that in the past. The spam that gets through now is a fraction of what is caught and that amount will continue to shrink as the filter is refined over time. There are occasions when spammers make life difficult but they will be added to the filter like the hundreds before them. The automated way is the only way to ensure the filtering is immediate because the human method is subject to timezones and response times: human = minutes/hours, computer = milliseconds.