Maybe it would have been best not to create a new value for the merit but simply use the activity the same as it is used for time and activity right now.
So instead of this
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
We would have used:
activity = min(time * 14, posts,
merit)
and everyone would have gotten the same initial merit as his activity at the time of the change.
This way nobody would have been affected in his current rank by a positiv or negative direction and we would still only use activity instead of an additional counter on all accounts.
But this approach may not stop the daily spam.
Also merit rating system can't stop really so much.
The spammer have a workaround for that solution just creating a couple of accounts and deliver the merit from them to few of their accounts increasing the rank of them.
I think only great content posts win.