As noted previously, the charedi community in Israel claims justification for its lifestyle from none other than Rambam. Rambam speaks about how the Tribe of Levi was devoted to spiritual pursuits and was supported by others, and adds that anyone can be like a Levite. Of course, as noted previously, Rambam did not mean that they can receive a halachic exemption from military duty, nor that they can receive funding from the community.
But let's put that aside for the moment, and discuss a different angle. What exactly did Rambam see such people - whether the tribe of Levi, or those emulating them - as actually doing?
Rabbi Dr. Jeffrey Woolf recently published a superb article in which he pointed out that according to Rambam, the tribe of Levi was not sitting and learning. Instead, they were teaching. As Rambam says:
Why did the tribe of Levi not acquire a share in the Land of Israel and in its spoils together with their brothers? Because this tribe was set apart to serve God and to minister to Him, to teach His straight ways and righteous ordinances to the multitudes, as it is written: “They shall teach Jacob Your ordinances and Israel Your Law” (Deut. 33,10).
In light of the immediately previous post, regarding the rationalist reasons for learning Torah, this is eminently understandable. Learning Torah is immensely important because it teaches theology, improves character, and perfects society. But it does not provide any mystical spiritual force-fields or anything like that.
Accordingly, only teaching Torah provides benefit to society, such that the Levites are supported by the rest of Israel. Learning Torah is wonderful for individuals; but they are not providing any benefit or service to society. According to Rambam - and, frankly, most other Rishonim - this does not justify communal support.