Yes, for a quick example. Some clans have it to where you attack the EB for gold and steal strip targets.
When attack builds assassinate in HTE does it reduce the personal plunder (profit made by the attack build himself)?
Yes and no. Assassination for an atk build pays him gold, but, it reduces the potiental gold he could make when you assassinate. Since the pay isn't so good, most clans require Atk builds not to use spies so the EB lasts longer.
No all actions help your plunder. However, it greatly decreses overall plunder of the eb. Clan makes less plunder. If the eb is long enough, ass will take away from attacks that you could do. Therefor, you make less potential plunder. So, if you have slow HTEs I advise no ass. If its flying, ass would help you but hurt your clan. What to choose is your decision however.
As an attack build, if you only have 1-3 spy buildings that arent towers you can hurt your personal plunder by using spies as well. This tends to mean you have spy attack less tjan 500k. Therefore wont apply if you have 3 max level Unkari Ice Trees.