What I like about L lenses (not in any particular order):
1) Build
2) Feel
3) Colour and contrast
4) Sharpness (most of them are very decent)
5) The feeling when I look at pictures taken with them (the "wow, this is awesome" feeling)
What do I dislike
1) Price
2) Can be heavy (some exceptions like the 70-200 4/L)
Do I have to buy? NO, but I have 2 specimens. Same type of question as:
- Do you HAVE to eat at Lei Garden when you can pay much less at a cze char place?
- Do you HAVE to drive a BMW or Audi when a Hyundai gets you from A to B?
- Do you HAVE to wear Timberland when Bata protects your feet?
- Do you HAVE to get a new 42 inch LCD TV when a 32 inch CRT works as well?
- Do you HAVE to get a iPhone when you can make calls on a Nokia 3120?
- Do you HAVE to wear a TAG Heuer when a Casio watch tells the time as well?
Notice I didn't mention astronomically expensive items (Rolls Royce, Zegna, Patek Philippe) but moderate luxuries
The answer is often about desires and wants and not needs. If you really talk about needs, then all we need is food, water and some protection from the elements.