According to many texts, the reason why Gringard reagents cannot be used with alcohols is because the Gringard reagent is a strong base, which would deprotonate the alcohol. ($p\pu{K}_b$ of Gringard $\approx 50$).
However, carbonyls also have a fairly acidic hydrogen at the alpha position ($p\pu{K}_a$ of enol $\approx 20$), so why does the Gringard reaction react with the electrophilic carbon, rather than as an acid-base reaction with the alpha position to form an enolate?
It actually happens sometimes and you end up with the enolate being formed in appreciable amounts. But it depends on other adjacent substituents that can withdraw or donate electron density to the alpha carbon.
If you draw the mechanism for the alpha attack you can see that the enolate can react further with the aldehyde and produce aldol condensation adducts. Furthermore, the carbon atom of the aldehyde is way more electrophilic than the hydrogen atom bound to the alpha carbon, that is the main reason why the enolate end up being formed in very small amounts and sometimes you don't even detect it.
