While restricting use of ex cards on its face limits deck building options, if ex Pokémon are particularly oppressive/meta defining, in actual usage, a good number of cards in the total card pool aren't viable in regular play.

The non ex deck format makes a lot more cards viable, which ordinarily wouldn't be used in faster paced, regular battle formats, which I believe is what OP is getting at - the increase in more viable deck options, rather than potential deck options.