Can white take advantage of this move order? usually black plays Nf6 before, but it allows stuffs like deferred exchange variation. So does it always transpose to main lines or it has some drawback?

It looks like it should transpose. White cannot stop black from playing b5 at any point after a6 has been played, so he doesn't really have a say in the matter if he wants to play the main lines. There is one independant line with an early b5 for black that I know of. After 5.Bb3 black plays 5...Na5 instead and goes for the bishop pair. This is called the norwegian variation and I think it looks better for white. Just something to look into...

So why i have never seen any game of grandmaster whis this move order? :) i just dont get it, but i also think that it should transpose, i know about Na5 variation but i asked about this because i want to play main lines more often (delayed exchange variation is very frequent, and i dont like playing it)

Alright so I just looked in my database and the most common move seems to be 5...Na5, although 5...Nf6 appears to just transpose back to the mainlines in most of the games. 6.0-0 is usually played, followed by 6...Be7. Then of course 7.Re1 is the mainline, but white can also try 7.d4. This probably doesn't lead to a better position than the mainline, but it is worth looking into. Statistically white seems to be doing quite well here though.