If it has suspension, then I wouldn't bother unless it has some expensive piece of top-end cycle components. The weight is one thing, the other is that suspension can (depending on pedalling technique) take a lot of the power you're trying to put into making the bike move.
If you can pick up cheap forks, it might be worth it but I put my searching efforts into getting a cheap-ish stiff bike for the roads and leaving the MTB for the days when I want to get dirty.