A good friend once told me: "every new lens you add to your equipment will devalue all the other lenses that you have."
Do I have to explain?
In other words: my shooting experience with a 50-mm-prime is so huge that I can survive any day and most jobs with one.
Any 24-mm-prime is of less worth to me because I only spent a few hundred hours with 24-mm-lenses.
Any hour I spend with another lens I do not spend to master the others.
85 and 50 are body parts. 35 ist growing fast on me.
All other stuff is specialized. Macro. Table Top. Repro. Panorama.
For me, there is a lot of merit in what Frank has said ... at least in mastering the lens ... the devaluation part is highly arguable.
When I was growing up, way way back in the film-only days, money and material things were much more precious and harder to obtain than objects of desire are now. One purchased a SLR with a 50mm. You mastered that 50mm, you shot the hell out of it until you knew that lens inside out, like it was your right hand. You knew exactly how it performed at f/2.8 or f/11, you memorized the FOV to where you knew the frame/composition without having to bring the camera up to your eye. After reaching a certain point of usage/mastering you felt it was time to move on and you purchased your next lens. (The whole time spent mastering the 50mm you were saving for the next lens.) You didn't need to go on a forum and ask strangers what your next lens purchase should be ... you knew after shooting the hell out of the 50 if you needed to go wide or long with your next purchase.
Once you decided on the second lens, you shot the hell out of that lens until mastering. Et Cetera. That was how you learned photography and built up your system. When I was working news, lenses were an extension of my eyes and the camera an extension of my hands. Manipulation of the settings was performed semi-automatically, never pulling my face from the viewfinder, concentrating nearly all my brain power on composition and capturing the story. Back then zooms were quite crappy, zoom lenses were like the bottoms of Coke bottles, so changing lenses in a fast moving environment had to be precise, there wasn't much room for error. Intimately knowing the FOV and DOF of each lens makes the difference between success or being scooped by the competition and getting your butt creamed by an editor, between keeping your job or getting fired.
I am a very strong proponent of what Frank is saying. It is a slow and painful process, but it does make you a better photographer than not mastering your equipment. Buying a whole lot of stuff is a lot more fun, but a lot of unmastered equipment dilutes the photographic process and will stretch out the learning curve.
While most of us here are hobbyists, the advantages of mastering a lens are not as valuable as they would be for a professional (especially when shooting news where speed is critical). But there is still a ton of value to be able to mentally compose your images at different FOV's and DOF's. To be so comfortable with your equipment that it is next to nothing to pick up and use a different camera. When how a camera/lens feels in your hand is the last factor you use when purchasing equipment.