It all depends.
If you ever play with others (other harpers who have levers on their harps, or other instruments) you might find that it is much easier to switch between keys quickly with levers. If you play for dancers, they won't want to wait for you to retune, and in a concert the sets have to be arranged carefully so that you don't have to retune too often. And what Paul said of course, switching keys within a peace is near impossible without levers.
On the other hand, levers are expensive, heavy, and need more maintenance than a plain harp. They should be fitted by someone who has some experience with the particular type of lever you want, but even if they really know what they're doing there might be a situation where you need to adjust them yourself, so if possible, have them show you how that would be done. Ill-fitted levers are just as bad as none at all, and they do get "out of tune" over time. If your harp is not new, it will have already settled, so this might not be a problem for you -- in fact, some harp makers even recommend adding levers only when a harp has been played for a year, for that very reason!
Levers also change the sound of the string. I know that every maker of harp levers claims that they have found a way that they don't, but I'm not convinced (and yes I have seen a lot of them). Every piece of metal between the wood and the string takes away sound, or at least influences its quality. Before you decide, have someone show you a harp with levers on it, sit down in a quiet room with it, and compare the sound and the feeling of the strings with or without levers. It always is a compromise.
If you're playing alone, you can transpose everything as much as you like, and I think many people with lever-less harps do so. You do change the range of a piece by doing that, though (you play it up to half an octave higher or lower than it is usually played) which changes the character of the music. When you want to sing to the harp, this might move song out of your vocal range (or the other way round, you'll need to transpose to some weird key to be able to sing it at all).
I'd say that for folk music, levers on the B, C and F strings are a good compromise. They are useful, but you probably won't need more than these.
not sure that helped