Arguably, it 'didn't hurt', especially if one assumes that the money and manpower were going to be spent on airplanes anyway. It is difficult to state definitively the effect, as it is commonly cited that German war production increased despite the raids. What is harder to determine is how much of an increase there would have been without them.

I'm most familiar with the figures for panzer production, and there is a definite correlation between the bombing of the cities in which the factories were located, and reductions in production. It should be noted, 'reduced', not 'stopped', as the inefficient German production methodology meant that a loss of production for a week was usually only on the order of one or two dozen tanks. The one notable exception was the effect on Alkett, which dropped monthly StuG production from about 250 to 50 for a while. (Made up for by an increase over at Krupp). Considering the US was churning out about 40+ Shermans a day, the overall effect on the war effort is relative. That said, there weren't all that many raids (maybe a dozen or two, if I recall correctly?) which specifically aimed at the Panzer production facilities, usually other targets were in mind.

Second-order effects, however, were ever-present. For example, the German manufacturing system was very distributed. The tank guns were made here, the engines there, the turrets over yonder, the hulls thither, and all brought together for final assembly in one place. The cities were also the transportation hubs. Hitting a railway marshalling yard in a city would reduce the ability to produce war material, and there are a number of documents in the record referring to supply problems, waiting on parts.

I'm not sure quite where the petroleum plants were compared to the cities. That was a very effective part of the bombing campaign, but I suspect that unlike the factories and transport, they were not really in the cities. Still, bombing cities might kill off a few of the workers and reduce productivity that way.

In any case, the strategic bombing campaign was definitely of some use.

Now, if your question is "was the attempt to bomb cities into oblivion as an end goal necessary", as opposed to "was it necessary to drop bombs in cities with the side effect of bombing the city into oblivion" (in the attempt to hit rail yards or whatever, bearing in mind the inaccuracy of bombing of the era), that becomes a far more iffy question I don't think I can answer.

(Readings for Panzer production from the works of Doyle/Jentz and Spielberger)