I have heard some people prefer to use frozen. Most of them are frozen when they are fresh and in off season it can be hard to get fresh otherwise. Also, things that have kinda a shell (peas, green beans) were easier to puree from the frozen form (I cook them a bit longer though too).
Also, sometimes... I have a hard time finding time to shop, cook, puree & freeze... with frozen they'll wait in the freezer for me! :)