The reason is cleanliness, avoiding a thin, porous scale deposit which would hold any contamination. Silicates do not cause corrosion of stainless steel although might indirectly contribute by holding salts and moisture against the surface. Also, if too much scale built up on a surface, there could be some abrasive effect if scrubbed off.
You can read about 'steam purity' in boiler books or in the Betz Handbook of Industrial Water Treatment. In order to avoid silicate deposits on steam turbines, the silica content of the steam is limited to 0.025 ppm (1/4 the value you cited, presuming measurement is of the condensate). This can be done by limiting the silica content of the feedwater (e.g., to 150 ppm for steam up to 300 psig, decreasing to 1 ppm for steam of 1500-2000 psig) or by use of an anti-foaming agent, which reduces volatilization. These silica values were taken from an American Boiler Manufacturers Association publication.
