If you have all your cards with one issuer [creditor] and there is a problem, you suddenly have no cards. If you have two creditors and one bails, your utilization may suddenly go up and this could negatively effect your score which may or may not cause other problems. 5 issuers and the behavior of one has less impact. Creditor=Issuer for purposes of this post.

There are 4 CC processing networks, AMEX, MC, VS & Discover. It is useful to have cards from least 2 of these, in case a merchant does not take cards from one or more of these networks. Example: Costco only accepts AMEX, Sam's Club doesn't take VS or AMEX credit cards. Many merchants skip AMEX due to merchant fees but VS is catching up with them with regard to fees and not being accepted.

It can be beneficial to have cards representing AMEX, MC & VS, as sometimes you can get perks by using a card from a particular network for a purchase. Example: US Air has a promotion going if you pay with a MC you can get a few extra miles.