Your looking at it from the aspect of what you want people to make. Not that that is bad or anything. But, you can look at it from what you personally, want to make.
I mean, if you want altars, than join and make an altar. I personally, do not want to make an altar. I would rather make a town hall type of building. See my point? While "Altars" limits it to a specific unit, "Buildings" limits it to a whole host of units. Luckily, "altars" fits in with the limitations of buildings. (I know you know this already, but I am getting to a point here.) So, obviously this idea will appeal to more model makers, which in turn will result in more models.
My point is, you can have one of two ways. 1. Have the contest theme be based off of what the public needs. 2. Have the contest theme be based off of the modelers want to make.
Neither is bad or good. It is just what the choices are. Hence, why we have a voting system. Just keep in mind, that the latter of the two, will obviously result in more models.
More modeling choices = more modelers = more models.
Less model choices = more choices of the same particular models (which results in the choice of which one fits best) = specific model needs met.
End results: 1. More models. 2. Less models. But specific. So it weighs out.
Yay for polls.