Do present weather observations (e.g. rain, fog, snow fall) in weather file have any impact on simulated performance of PV solar panels and solar collectors ?

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I don't think on general principles that EnergyPlus should incorporate climate modeling capabilities, as that could easily lead to double-counting and likely misuse (site-specific effects like shading from neighboring buildings and wind reductions due to terrain-factors excepted). I'm not so concerned about certain parameters on the weather file not being used in deriving solar radiation. That doesn't mean their effects are ignored, just that they're expressed in other parameters, chiefly the Sky Cloud Cover. When there's rain, fog, snow, etc., the Sky Cloud Cover should be 0. You might argue that Cloud Cover 0 + Rain, Fog, etc. should reduce Solar Radiation even more, but how much more? You'd need to get some good measured solar plus weather station observations and then add those additional terms to the regression which, if you're using Z-H, already has 4 (Cloud Cover, Temp Rise, Wind Speed, and RH). I have no idea how much would be the improvement, if any.