I think it's useful to at least get a feel for other systems just to increase your overall experience (and there is the added benefit of finding out the trickiest or most annoying responses from my opponent's side so that I can use them myself), and if you're having a hard time at some point then changing openings can give you the feeling of a fresh start which is a psychological plus. Also, if you play tournaments then playing the same stuff all the time makes it easy for your opponents to prepare against you, not only can they save a lot of time on the clock but you can be pretty sure that for many opponents you will be playing against their 3000 elo engine for the first 10-15 moves. One other point is that with a wide repertoire, occasionally you may recognise an opportunity to transpose into a favourable variation of another opening that you studied (maybe this doesn't apply so much to Sicilian but certainly there is French/Caro/Scand overlap for example). So yeah, play everything, ok maybe not unsound guff but you get the idea :)
Is the Sicilian the only e4 defense I need to know?
I think it's impossible to say what reply is opjectively the best. The positions you are most comfortable in should determine your choice imho. Do you like open sharp games or more positional games. A major opening you didn't mention in your post is the very popular 'Ruy Lopez'. Like 'The sicilian' it has a ton of theory, which is why I don't play either of them 
Yes, it's fine to limit yourself to just a single defence to 1.e4.
the whole idea behind the sicilian is kind of a mysterious one. if you're starting out, 1..e5 might be the way to go.
Presuming you only play 1.d4, 1.Nf3 or 1.c4 and never want to use 1.e4 then yes, only learning the sicilian is possible.
I'm not sure if it's objectively best and personally I couldn't prove it anyway. It's better to just play what you like...
no....u should know the french defence too...
No, there are still many others that you should have known.
1. Alekhine Defence.
2. Scandinavian Defence.
3. Caro Kann Defence.
4. Ruy Lopez.
5. Scotch Game.
6. Petrof Defence.
7. French Defence.
i dont think there should be any problem in it. The more games u play in an opening the better u ll get hang of it.1...c5 is an adequate response to e4, whatever be the second move of white's.
If u get bored then instead of turning to the heavy guns, e5/c5/e6/c6/g6, u can choose other simpler opening, without disturbing ur comfort level with the sicilian. like center counter gambit or alekhine's def. these openings are based on simple ideas and i dont think any opponent u might face at our level will be loaded with theory.
my experience with different openings(agianst e4)--i got destroyed in pirc when i tried it so i have abandoned it. while now i play french very rarely with a completely diff style than i used to play. i have shifted from c5 to e5, as my major response. i got owned in caro-kann and never tried it agian.
ruy lopez is not a defence.....
In the long run it's detrimental to your development if you don't study all of the classic openings. If you go and talk to any strong player you'll most probably find out that they know at least some basics about every reasonable opening. The main point of studying openings isn't learning specific variations but rather learning to undestand the standard plans that apply for the typical positions arising from them and It's very important to be comfortable in many different type of positions.
Also, even if you play only Sicilian you are bound to from time to get positions that more closely remind some other opening. For example in open Siclian black often has the chance to play d5 and white can reply with e5 when the position reminds a French Defence. Then it's obviously very usefull to have at least some familiarity to French both in order to play the arising position better but also to understand whether it's a good idea to enter into such structure in the first place. On the other hand, it's probably very advisable to mainly play one opening at a time to really learn to understand it.
On another note, the fact that Kasparov and Fischer both played Sicilian is probably one of the most common reasons why many amateurs use it and may also be one of the main reasons why I picked it up but thruth to be told it's really bad way to choose an opening since it may well be that what's suitable for Kasparov or Fischer is entirely unsuitable for them. Of course for a begining player it's probably impossible to decide what opening would suit for their style in any case
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Of course for a begining player it's probably impossible to decide what opening would suit for their style in any case.
The one in which he wins and is comfortable going against a higher rated opponent.eyerone gets used to an opening for some time.
If you are playing the Sicilian, yes, this is the only defense you need to "know".
Now, just "learn" it. I think it takes a bit more time than you think.
You should know the french defence
technically you really only need to knwo one - a second one may be useful for surprise or maybe u could just go with the dragon once then the nardoff then the main line - so ya you dont need to spend time learning otehr defenses.
PS - If you want to learn about teh sicilian then read - chess openings for black explained
Sicilian is too hot blooded 4 me. I like a more positional opening such as the Caro-Kan. Maybe its just a matter of taste.
... or maybe u could just go with the dragon once then the nardoff...
Is nard off a chess opening? I thought it was an insult.
hahha the nardoff defense that is rich
against e4 I only play the sicilian..... if this is objectively the best reply to e4, is it worth it to learn other openings such as the caro-kann/scandinavian/alekhine? i looked through kasparov/fischers game on chessgames and they pretty much just played c5 exclusively against e4