The thing about owning a meter is...learning to see light.
The thing about owning a spot meter is...learning to see light precisely.
Most meters (incident) measure an average of light - the amount falling on the subject. If you were shooting a mountain scene and the light falling on you is the same as on the mountain, incident would work fine. If the light wasn't the same, you would have to compensate (if you knew how many stops difference between "your" light and "it's" light.
Spot meters read reflected light - so if the light on the mountain isn't the same, you could still get an accurate reading. The difference is that the spot meter is designed to give you a reading ONLY where you pointed it at. Your camera is reading reflected, but usually has some form of tweaking internally which doesn't always give you a dead on measurement.
HOWEVER - all light meters (including a "neutral" in-camera meter) are programmed to read medium grey (13 or 18%, depending on what camp you come from). If you pointed a reflected (spot) meter at something black, it'll give you the readings to make it medium grey. If you pointed it at a white cloud, it'll give you the readings to make it...medium grey. You eventually learn that pointing it at grass (though it's green), is about dead on medium grey.
The other thing you learn is that certain things are "x" stops off med grey - so, pointing it at dirt will give you a reading. You learn that it's about a stop lighter (or darker) and so you alter your spot meter reading. It sounds hard, but in the end, you begin to "see" light. Hell, I can dial in exposure on my Hasselblad most times using my eyes and I'm pretty damn close. There's a chart in the Hasselblad Manual that shows the equivalent colors to medium gray and how many stops others are off!
The nicest thing about a hand meter is that it allows you to "read the scene" by taking different readings of highlights/shadows/major details and allowing you to determine which way to bias the exposure. A in-camera meter will give you an average. So will an incident (domed), assuming it's the same light falling on the scene as on the meter. That average isn't always best, so you can bias it.
In studio, it allows you to get the lighting +/- .1 stops if you wish. It also allows you to get ratios by pointing the dome at each light.
Instead of spraying-and-praying with the camera to dial in the settings (and even then, it's probably +/- 1 stop), you can use the meter to get your lighting dead accurate for the 1-shot, dead-nuts-on exposure. It's one less thing to worry about.
The 478 is a cool meter, but if you REALLY want to learn lighting (especially if you ever want to learn to do B&W correctly - Zone System), you HAVE to learn to use a spot meter correctly. The touch screen is nice, but a bit gimicky. I like my 758 with spot - I have an additional display inside the spot so I don't have to even take my eye off the scene. I can hold down the button, and sweep my scene, watching how many stops (of dynamic range) is in the scene. That helps pick ND gradients for landscapes. It also helps me dial down/up lighting for studio work.
I also own a Minolta Color Meter - so I can get the color temperature correct up front. If I have some lights that are ~500K off, I can use a gel and get all the lights to match. My CM will tell me what correction I need (not that I haven't already memorized those corrections!).
Finally, I'll shoot a MacBeth in the shot. Then, in LR, I can correct for the color temp using the colorchecker. I can already be sure that all my lighting is even (or not, if I wanted certain lights "x%" off). I can also be sure that my whole scene is within the range of capture (or if blowing out something purposefully, be sure that it's the only thing blow out - or blocked up, if that's what I intend).
With all that said, I don't get why a lot of people "only use the camera in manual mode". It sometimes makes for extra work that isn't needed. A good hand meter can also teach you the quirks of your in camera meter. And, you should learn to trust/use the in-camera and how it reacts to different environments.
I use my 1Ds3 in Av almost all times (unless I'm in studio w/ strobe lighting, obviously). You eventually learn how the cameras meter reacts, when to dial + or - compensation, when to use averaging/matrix/weighted/spot, and when (really, where) the meter stop being accurate (low light, high light, edge light, etc). If the lighting gets rough, I pull out my hand meter and go to work analyzing the scene. I may end up leaving the cam in Av, but using spot/weighted mode and aiming it at a specific location (determined by the hand spot meter). Or, I may go completely manual then and determine an exposure based on what I spot read from the hand meter.
Spend the extra money on a 558, 608 or 758 - it'll be better than the 458 if you REALLY want to learn light and get dead accurate exposures. I used to strive to shoot *1* polaroid on my Hasselblad/large format to confirm my exposures. I still try to only shoot one "test" frame on my 1's to be sure of the exposure before a "final" - keeps that shutter life down (not that it's really an accurate number anyways!)
I shoot about every subject in the photographic realm. Some days, I shoot portraits. Another may be an architectural day for a client. Right afterwards, I've been known to go shoot myself some neat landscapes. I also do commercial/product shoots. I hate weddings, but have shot too many - so I'll shoot 1-2 per year just to stress myself a bit (not really, they're fairly easy once you get used to it). My hand meter always accompanies me - and has been all around the world on vacation in my back pocket (more so back in the days of vacationing with a Hasselblad). I prefer not to "shoot pictures" - I'd rather "create" them.