Father Dubay says that he often runs across an attempt to define voluntary poverty as "availabity" -- generosity with one's time, energy and talent. He says that this self-donation is praiseworthy but not identical with voluntary poverty per se... and that a study of the Gospels will show that when they refer to voluntary poverty they are talking about material goods, not personal availability.
The way I understand it, availability or personal generosity can overlap with poverty but are not the same thing.
The idea seems to be that redefining poverty as availability might allow niches where human nature can creep in. So one could say one was poor in spirit because one was available even though still placing too much value on possessions and money.
Plus, perhaps too much value placed on human wealth can make a block which makes it hard to be truly available. Riches bring: (1) cares and solicitude (2) love of riches and (3) vainglory or elation, according to St Thomas (see below). He particularly notes the first one as being the main problem with people who aren't particularly rich. The second two are more characteristic of the truly wealthy. And certainly you can think of lots of examples where people make life choices dependent on getting more good things -- like the archetypical dad who neglects his children while pursuing extra hours at the office "for their sake". Aquinas says that there is nothing wrong with taking normal care for provision and security, particularly when one is responsible for a family -- just getting enough to live simply on does not hinder perfection.
So he says that "doing away with riches" removes certain obstacles to charity -- and makes it easier to have a truly generous attitude.
He also says, as Father Dubay says in a later chapter, that it's not poverty ITSELF that is good -- it is a means to an end. More on that later.
The Catholic Encyclopedia, in its article on Poverty, says:
Cupidity, vain glory, and excessive solicitude are, according to St. Thomas, the three obstacles which riches put in the way of acquiring perfection (Summa, II-II, Q. clxxxviii, a. 7). ...Thomas Aquinas says:
The warnings and counsels of Jesus Christ are valuable even to those who are not vowed to a state of perfection. They teach men to moderate their desire for riches, and accept cheerfully the loss or deprivation of them; and they inculcate that detachment from the things of this world which our Lord taught when He said, "Everyone of you that doth not renounce all that he possesseth, cannot be my disciple" (Luke xiv 33).
... perfection consists, essentially, not in poverty, but in following Christ...
Now the privation of one's possessions, or poverty, is a means of perfection, inasmuch as by doing away with riches we remove certain obstacles to charity; and these are chiefly three. The first is the cares which riches bring with them; wherefore our Lord said (Matthew 13:22): "That which was sown [Vulgate: 'He that received the seed'] among thorns, is he that heareth the word, and the care of this world, and the deceitfulness of riches, choketh up the word." The second is the love of riches, which increases with the possession of wealth; wherefore Jerome says (Super Matth. xix, 23) that "since it is difficult to despise riches when we have them, our Lord did not say: 'It is impossible for a rich man to enter the kingdom of heaven,' but: 'It is difficult.'" The third is vainglory or elation which results from riches, according to Psalm 48:7, "They that trust in their own strength, and glory in the multitude of their riches."
Accordingly the first of these three cannot be altogether separated from riches whether great or small. For man must needs take a certain amount of care in acquiring or keeping external things. But so long as external things are sought or possessed only in a small quantity, and as much as is required for a mere livelihood, such like care does not hinder one much; and consequently is not inconsistent with the perfection of Christian life. ... Yet the possession of much wealth increases the weight of care, which is a great distraction to man's mind and hinders him from giving himself wholly to God's service. The other two, however, namely the love of riches and taking pride or glorying in riches, result only from an abundance of wealth.