I guess I can see that argument, but at the same time its so easy to add memcache that it really seems silly not to add memcache for you models at are accessed a lot. Here is a simple user model, adding memcahce is a total of 7 lines. I just cant see the case where you wouldn't add this to start with:

  from google.appengine.api import memcache
  from google.appengine.ext import db

  class User(db.Model):
    email = db.StringProperty()
    password = db.StringProperty()
    sessionKey = db.StringProperty()

    def userKey(self):
      return "User.session="+self.sessionKey

    def put(self):
      super(User, self).put()
      memcache.delete(self.userKey())
      memcache.add(self.userKey(), self, 600)

    def getCurrent(self, request):
      user = memcache.get(self.userKey())
      if user is None:
        user = User.all().filter('sessionKey = ', request.header.cookie.get('user'))
        memcache.add(self.userKey(), user, 600)
      return user