i have the 70-200 f4L, the 135 f2L and the 85 f1.8.
I do not feel that the 135L "overlaps" the 70-200. yes, the focal range is covered in the 70-200, and yes, the 70-200 gives me great pictures, but the 135 f2L is just totally in a different league in terms of picture quality. it's got that dreamy pop that no other lens i have can give. i still use the 85 f1.8 when i want prime quality sharp pictures even though i know the 70-200 is in my bag. options i guess.
maybe it's just me, but when i shoot with my 135L, i get a totally different feeling that shooting with the 70-200 f4L doesn't give. they're pretty close in price too so price is not the reason. i bought this lens to learn to shoot with a tele prime, and the 70-200 comes out when i have no space to move around, or if i know i am going to shoot a variety of situations. for fashion shows i am usually standing at one spot so i will just pick 85 or 135 depending on where i am standing at that show in particular. i love primes, and the 135L is just one heck of a nice prime to use just for the fun of it.
If forced to answer, no one really NEEDS a 135 f2L for a hobbyist. Heck, you would not even need more than a pns. If you want it and you can afford it, go for it. You will enjoy it a lot, and if you really regret it, the resale value is very decent.
i have a few different types of screwdrivers and wrenches in the same size, so i don't need a t-spanner, but it's a nicer tool to have if you're working on a part that is hard to reach with a normal socket or crescent wrench. sometimes having the perfect tool for the job just makes you feel great working with the tools you know?