It isn't quite as simple as yes or not but as a general rule; yes, dismantle all your old armour and weapons. There are two exceptions; firstly for crucible. In regular crucible your light level and weapon attack values are 'normalised' so a level 5 with a 50 attack gun can kill players just as well as a level 40 with a 300 attack gun. Therefore if you find a gun you like for PvP then you can keep it even if it is useless in PvE. The same applies for armour, the armour that lets me be light level 300 gives me too much strength so for PvP I switch to older armour that is intillect and discipline as light level doesn't matter. Another reason for crucible s if you like the perks on the armour, again for PvP I have kept an old set of boots as it allows me to carry more rockets. The second exception you will get to later on and is around infusion of armour. You might get a 260 light legendary armour but a 270 blue might drop however legendaries are generally better; they have more perks on and generally higher figures for int/disc/str than blue's. So instead of dismantling the legendary you can infuse the blue in to the legendary to make the legendary to 270 light. This doesn't work exactly, a 280 blue infused in to a 250 legendary will only bring the legendary up to 268 or something.