That argument always comes up and I've never understood the logic. A player who grinds up to an Anaconda only in Solo mode is learning to fight NPC's. They'll lose that ship very quickly when they finally switch to Open and meet players who have been learning how to fight other players.
If that Anaconda player has developed no contacts with other players while in Solo mode, he'll also be facing Wings in Open mode soon enough. You don't learn anything about multi-ship tactics in Solo mode.
In short, there is no advantage to getting your Anaconda in Solo mode and then switching to Open for pew pew action.
Only if you're up against another Anaconda or a Python. If you're a solo trader who's jumped into open with his Anaconda and you're up against the most talented Viper pilot in the world unless you really are just absolutely awful I just don't see how that lone viper pilot would be able to break through the anaconda's shields before succumbing to its superior firepower.
I think most people playing the pirate or bounty hunter route generally stick to open, they may have more combat experience but they've also picked the "career path" with the least rewards, at the very best you might see one in an asp, but you're much more likely to see them in Vipers and Cobras. They would get curbstomped by a solo trader in a Python or an Anaconda.