Personally, playing casually I have no problems going for 5-6 players even without a second set of basic cards. Usually, we include 12 Kingdom cards in these games - mainly so that there are substitute cards for piles likely to disappear quickly. The biggest downside is that you can't just play full random like this, you kinda need to control what's going into the game to avoid some horrible things - some attacks, things that ram through treasure piles, make sure there's no kinda 'one pile is absolutely key' cards, stuff like that. Normally, it works, but having to go through that effort while setting up obviously makes it impractical generally.
And I agree, if you're with 5+ players and have twice the basic cards you should just play 2 games of Dominion, the 5-6 player rules are most useful for this puzzle subforum
Most groups don't like to split into two games, even if lots of people want to play Dominion. With the veterans, sure, but with casual players (which most people in the world are), you're just going to play with 5, not split into two.