I always my reservations when it comes to this statement and it relationship with investing in more than one broker. In as much as I believe in diversity in investment, I also think that if one gets a good and regulated broker, then there is no cause for alarm.
Sure, we can go unlimited when we start naming the regulatory bodies but depending on the country that the broker is or have extension in. Like those that are in Cyprus (CySEC) though I don't know how strong their regulation is that makes brokers always love to go there,, MiFID that covers...
Personally, I do not see anything wrong in having so many brokers on the queue. I see the competition as that which makes the business fun this way because it subsidizes Forex for the poor and average Forex trader who may not want to go so low when it comes to minimum deposit.
Of course, the presence of so many many brokers has really made the business a very competitive one. Traders these days now have the option of choosing from a variety of them and that is a plus for the Forex market as it makes it more popular.