The issue with silicate and RO/DI is that the uncharged form (silicic acid) gets through an RO membrane to some extent, and silicate is among the most poorly held ions on a DI, so if something else comes along and displaces it, it can pop off and be released.
Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com
However, at the small end of the spectrum a number of compounds can pass through a reverse osmosis membrane to some extent and are, therefore, of concern to reef aquarists.
These include carbon dioxide (CO2), ammonia (NH3), hydrogen sulfide (H2S, especially a concern with well water) and silicic acid (Si(OH)4, which is the uncharged and predominate form of silicate at pH values below 9.5). All of these should be trapped by a functioning DI resin (discussed below), but can still be a concern.
In the case of CO2, for example, there can be a lot of it in certain well waters, and DI resins may become rapidly depleted because the CO2 so readily passes through RO membranes (how to deal with this is discussed later in this article). As another example, ammonia that comes from chloramine in the water can be significant, and is one reason that RO/DI is greatly preferred to RO alone in those situations where chloramine is added to the tap water.
In the case of silicic acid, some types of RO membranes can be better than others at excluding it, even before it gets to the DI resins. For example, a thin-film polyamide membrane might let only 0.3% of the silicic acid pass, while a similar cellulose acetate membrane might let 12.7% of it pass.
then later...
In the DI descriptions above, I did not address the fact that some ions will show a greater preference for attachment to the resin than will others. When the resins are not depleted, it does not matter what the ions’ affinity is, as all are bound. But in a depleted scenario, when there are more ions than ion binding sites, those with a higher affinity for the resin will be retained, and those with a lower affinity will be released.
It turns out that silicate is found at the lower end of affinity for anion resins. Consequently, if the DI resin has been collecting silicate for a long period and is then depleted, a large burst of silicate may be released.