In this section, Moldvay says:
"The DM should try to maintain the "balance of play". The treasures should be balanced by the dangers. Some groups prefer adventures where advancement between levels is swift. In such a case, since the treasures are generally greater, the monsters should be "tougher". Other groups prefer adventures where character development is more important, and advancement is slower. If the monsters are too tough, and if the parties are reduced by many deaths, then few characters will ever reach higher levels."
Most of this seems like common sense, so I don't really have a lot to elaborate on here.
It is interesting though, that Moldvay reminds us that there are higher levels there to be attained and that the DM should keep this in mind when plotting out adventures and campaigns.
I'm curious - how important is "leveling up" in your campaigns, as opposed to more in-depth character development?