Having more third parties adds in a new political problem.
Take the UK as an example. Their parliamentary system sometimes suffers because there are several different major factions in play and these factions often make and break alliances on certain issues and the focus is often on infighting between different groups.
More political groups often means more fighting and dealing for allies rather than focusing on issues. A deadlock is also a more frequent occurrence on issues
Not saying that third parties are bad, just that they have their own political hangups and are not a magic cure for political gridlock.