As long as you know what the heck you're doing, (sound) sidelines are better. If you know the theory behind the b3 sicilian and your opponent doesn't (because it's not mainline, so he has less chance of studying it. no one can be prepared for everything), that's an advantage for you, and if he does know what he's doing you can still play winning lines in that variation and get a winning position.
Chess Talk: Mainlines or Sidelines for Practical Play
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I just play the line that understand the most sometimes it's tha mainline sometimes it's the sideline
Practically, it depends entirely on which variation you know and understand better.
Assuming that this isn't the issue at hand, then I'd have to agree with RyanMK. Side variations often include moves that are taboo in the mainline and could lead to your opponent overextending in an attempt to capitalize on your "mistake".
As a counterpoint, however, if you play something out of style the chances of staying in book moves for very long aren't good and you'll find yourself having to work a bit harder as well to make the best of your opponents passive moves(if they're at a loss for a plan).
hey KB, I bet I can guess which you prefer... 
Actually, I am a fan of main lines, but am considering playing a few sidelines as my main repertoire.
KillaBeez wrote:
Actually, I am a fan of main lines, but am considering playing a few sidelines as my main repertoire.
That's news to me. What about your novelties, what you said about main lines being predictable and variations being surprising being reasons you momentarily switched to e4?
I actually play a mixture of both. How about we discuss the merits of sidelines versus mainlines and not worry about what I play.
Knowing main lines will make you a better player in the long run. In the short term, sielines are great but used to often your easy wins can dry up. Well, that is a problem if you play OTB against the same people (they wont fall for things twice), for example, tournaments in your city will usually consist of more or less the same people. However, if you are an Internet player you wont have to worry about that as you face different people every day.
It is a good idea to base your repitoire on main lines and then learn some extra sidelines. It is extra work but it will pay dividends. Think about it, if you learn the main lines you can add sideline ideas easily to your knowledge. It is more difficult to rely on sidelines and then try to learn main lines. Generally, it should take more time and effort to learn main lines thus your repitoire would be less flexible without knowing them.
Mar 10, 2009
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#10
If you play sidelines, you have to surprise your opponent and undertand the positions better.
If you play mainlines, you have to know the opening better and reach the kind of position you like.
Mar 10, 2009
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#11
Catalyst_Kh wrote:
(snip)
Main lines leads to undestandable positions with well known possibilities, that you can handle even in face of stronger opponent.
I don't agree with this. Main lines lead to the most rich and complicated positions, not really the positions where it's easy to find your way...
See for example Najdorfs or Botvinnik semi-slav - not easy to find your way there if you're not already extremely familiar with the positions
I was just looking for responses to the following question. For practical play only, do you think that it is better to play sidelines (ex. b3 Sicilian) or to play main lines of openings. I am interested to hear your opinion on the matter.