Aye, 99% of the time its not an issue, but when we went on our honeymoon, I put the hotel on my visa, and then I used my visa for the car rental (was only 80 dollars for 3 days, but they but a temp authoriz on it for 300), so I had no more limit (even though I did), so when we went out to eat the second night my visa got rejected, and they didnt take discover.... luckily my newly acquired wife had a visa with another 1k on it so we used that (everything was maxed out for the week before/after the wedding).
Anyway, I know we dont 'need' another card, but since I am ignorant of how credit works for the most part, I would think dabbling in all the major credit cards at least once for a little bit would be good.... but thats my flawed logic probably... since they are only supposed to look at your credit report, and not at how good/if you used them before.