Ne1 and f3 can be playable - often it's quite risky to weaken the g1-a7 diagonal like this. Judge if it can be exploited.
Taking the knight is often acceptable. Notice that by playing d5 and f5, Black hands White the opposite square e5 for one of his own knights. Try installing a knight ther yourself, and see what Black does about it.
1. d4 Nf6 2. c4 e6 3. g3 d5 4. Nf3 Be7 5. Bg2 Nc6 6. O-O O-O 7. Nbd2 Ne4 8. Qc2 f5
I see this kind of setup resembling the stonewall quite a bit. Installing a knight on such a great square has obvious advantages and by taking it white gives black a strong central wedge. So, how should one deal with this kind of strategy? Nf3-e1 and f2-f3? Try to undermine by attacking e6? Or just accept the knight and keep playing? If the latter, what compensation does the knight jump imply for white?