It also doesn't mean you can fly outside the SID. A lot of people increase speed to 290Kts or similar when there is a tightish turn and end up increasing the turn radius so much that the aeroplane can't fly the SID. I like to keep the speed back until the turn has been completed, it also decreases the noise (power comes on at a greater altitude, or when level).
Manchester and Edinburgh now fine airlines for poor track keeping (not something I like), but we should fly the SID accurately.