Well, it's debatable of course, like anything in this hobby it seems, but typically sponges are a sign of good water chemistry and overall system maturity, imo. Although they are formed from silicates, their presence is not necessarily a sign of an excess of silicates. They are fine filter feeders. A gifted reefer once told me that SPS won't really thrive until sponges are present. In my ignorance and/or naiveté , I've just trusted that.
Some sponges can be sort of invasive, taking over zoa colonies for example, and crowding out other things, but not for the most part, ime.