I think that totally denying public funds to the homeless is far from being feasible as it is part of social security, which in turn is part of the Universal Declaration of Human Rights:
Everyone, as a member of society, has the right to social security and
is entitled to realization, through national effort and international
co-operation and in accordance with the organization and resources of
each State, of the economic, social and cultural rights indispensable
for his dignity and the free development of his personality.
Social security/protection is also part of the Inclusive growth concept.
From a layman perspective (as I understand it): we do not plan for letting people die in the streets. We do something about it.
"Some people say panhandling yields significantly more income than most people make by working."
Although I have heard about such rumors, I think this is highly exaggerated in most of the cases (at least within Western countries). The first answers from here provide references related to income and expenses of beggars. One particular study is this and its conclusion is the following:
[..] Toronto-centric study considers both numbers, and it concludes
that the majority of Toronto's beggars live in extreme poverty and are
homeless.
The same article explains the complexity of the phenomena and suggests that it is not only the financial dimension:
The homeless often need something more than money. They need money and
direction. For most homeless people, direction means a job and a roof.
A 1999 study from HUD polled homeless people about what they needed
most: 42% said help finding a job; 38% said finding housing; 30% said
paying rent or utilities; 13% said training or medical care.
This article deals with the question from the title and its conclusion is the following:
The short answer is no [, we should not give money to homeless people]. The long answer is yes, but only if you work for an organization that can ensure the money is spent wisely.
This article deals with a success story related to homelessness and confirms that a more complex approach (not just financial aid) gives the best results:
Initially, critics feared Utah would lose tons of money by giving the
homeless permanent housing, and that doing so would just "incentivize
mooching," as Minhaj put it. However, state officials found Housing
First actually saving the government money over time, especially as it
encourages people to become more self-sufficient sooner.
Moreover, Housing First homes are not free: New tenants have to pay
$50 or 30% of their income to rent each month (whichever amount is
greater).
Between shelters, jail stays, ambulances, and hospital visits, caring
for one homeless person typically costs the government $20,000 a year.
Providing one homeless person with permanent housing, however — as
well as a social worker to help them transition into mainstream
society — costs the state $8,000, The New Yorker reported in
September.
"Is there any validity to this argument?"
Clearly, taking care in any form about the homeless incur costs and I find the only reasonable argument for denying public funds. But, as already illustrated, careful spending can provide good results.
So, yes, there is some validity, but I would say a little one.