Nowt wrong with just cards.
I've sat and watched Ed Marlo, Guy Hollingworth, Lennart Green, Simon Lovell, Sal Piacente and many more give such creative routines using just cards. Pure entertainment. Solid sets. And more than just this, fantastic skill. Not a sponge ball, coin or hanky in sight.
So many magicians will recommend you vary your set a little—but if you're comfortable with sitting down with a deck and entertaining a crowd then why try to change?
Some cardies use gimmicks or props in their magic (for instance, making a shoe appear from a cut deck, Lennart Green). But these are part of the card routines.
The advice would be to analyse your performance AND your target audience.
Kids don't appreciate the skill or quality of card magic—they're preferred magic is much more visual and innocent. More mature audiences generally are warmer towards cards, and you need to consider audience participation as being paramount to the entertainment value.
I would be prepared to wager that if you took all other genres of magic and pitted them against card magic alone, you'd probably find that card magic dominates to some extent the field of magic.
And because of this, it may seem conditional that a magician is able to perform card magic. But this is more an assumption than a criteria.
Being able to perform a card trick or two is handy. But being able to create a whole hour long set from just card magic is a real skill.
In conclusion, I'd say that it's totally up to you whether or not you add additional material to your card set. But I've said it already once today, if it ain't broke—don't fix it
