There's no clear and unequivocal answer to this issue--the impact of leadership assassination (whether by drone or otherwise) is entirely contextually dependent.

On the positive side, it not only causes attrition of those who (may have) shown considerable skill in order to rise to their current level of command, but it also frequently causes a degree of internal paralysis as the organization concerned assesses how its security might have breached. In highly compartmentalized organizations, some information may be lost with the target. It may also send signals about who has the upper hand, or about the reach of the instigating country/organization, that have morale effects on the organization or the general population. It can deter.

On the other hand, it can backfire. It can be seen by the local population as a violation of local sovereignty. It can cause radicalization. It can be used as an aid to recruitment. It can have unforeseen second and third order effects. It can result in even more skilled leaders emerging, as Fuchs notes (Hizbullah/Nasrallah being a case in point). It can cause ###-for-tat killings, an expansion of the geography of confrontation, or uncontrolled escalation.

I can think of cases where the death of a leader had devastating consequences on an organization (Savimbi/UNITA, Zarqawi/AQI). It would have been pretty good too if someone had taken Hitler down before 1945. I can think of cases where assassination paid major operational dividends, but also somewhat strengthened the target organization politically (Ayyash/Hamas, Mughniyah/Hizbullah). There are cases where the operational effect was marginal, but the countervailing PR gains for the targeted organization were substantial (Israel's 1973 assassination of three PLO leaders in Beirut; Yassin/Hamas, or for that matter the PFLP assassination of Rehavam Ze'evi). There are cases where the diplomatic cost of the assassination outweighed the potential gain (Mishal/Hamas). There are cases where the short-term operational effects benefited the attacker, but there were unforeseen longer-term effects that offset these (Khalil al-Wazir/Fateh).

The point here is that this where diplomats, intel analysts, politicians, and perhaps even lawyers ought to earn their pay by doing some solid analytical thinking about costs, benefits, precedents, and contingencies, rather than reflexively adhering to the position that leadership assassination is always "good" or "bad."