Accumulation is the advantage of this system : you are more flexible,you can play some moves faster and if you need more time for a complicated position or if you have no time at home you can use some more days for a move. But you are right, there should be an additional limit and I see here two possible solutions:
1.Accumulation is only possible within one period,for instance 10m/10d it begins new at the 11th move..or

2.Accumulation is possible through all the game,but the time until the next move could not be more than 10 days (or after the 15th day without a move it is a "tacit recession" (the right word"?:))

I find the first point a little bit better.
Btw in the past we played correspondence chess with postcards 10m/30d was standard (without transport,incl weekends),now in the web you will see this is much,much to long..