I am glad you asked this, because I have been thinking about it myself, as my old Mamiya-Sekor's internal light meter died but I want to still be able to use it, since it was my first SLR that I got in the 8th grade and is special to me. But I know nothing about light meters...
if you have ttl metering that is capable of spot, center, and matrix metering, then you should be well off.
incident meters are good if you are a stand still photographer who shoots stand still subjects in forever even lighting.
a tech junky who should shoot more than do math may jump in and argue the 12.5% gray vs. the 18% gray issue, but it doesn't mean a rodent's buttocks if you know how to make a one time slight adjustment to exposure. reality is that most ppl couldn't see the difference.
i've owned 2 handheld meters. i gave them away. if you don't have spot metering ttl, a handheld spot meter would be a nice addition.
I use a Gossen Lunasix F ..... got it at the same time I bought a used Mamiya 645. so I use it with that also with an old Nikon FM whose inbuilt meter is way out of whack. But if I did not have either of those cameras I would not bother with a hand held meter.
The meters in the new cameras are better than ever, but still not as a good as incident metering. If you have an older camera with an inoperative meter, they are worth it. I have a Shepard meter that cost me a little over $100 and it is very accurate. I use it with my Yashicamat. You don't have to spend 200-300 dollars for a good light meter. I think they are even more valuable if you shoot slide film.