The point that Holden is trying to make when he elaborates about suitcases is that people judge others by the kind of suitcases they carry.  He says,

"...it's really hard to be roommates with people if your suitcases are much better than theirs - if yours are really good ones and theirs aren't.  You think if they're intelligent and all, the other person, and have a good sense of humor, that they don't give a damn whose suitcases are better, but they do.  They really do".

Holden is talking about prejudices that are deeply ingrained in people.  Having a nice suitcase identifies an individual as belonging to a high social class, something to which most people aspire even while pretending that they don't.  Holden remembers the example of an old roommate at Elkton Hills, "Dick Slagle, that had these very inexpensive suitcases".  Dick used to hide his suitcases, even as he made fun of Holden's expensive suitcases for being "too new and bourgeois".  Feeling bad for Dick, Holden began to hide his own suitcases so as not to show his roommate up, but Dick kept taking them out again.  As it turned out, Dick wanted Holden's suitcases to remain out on display so that people would think Holden's bags were his. 

Ironically, even though Holden says he hates the way that suitcases seem to define people, he himself is guilty of the same prejudices.  When he sees the nuns come in carrying "very inexpensive-looking suitcases", Holden is repulsed.  He says,

"It isn't important, I know, but I hate it when somebody has cheap suitcases.  It sounds terrible to say it, but I can even get to hate somebody, just looking at them, if they have cheap suitcases with them".

Although it might be argued that Holden's aversion to people with cheap suitcases stems from his experience with Dick Slagle and so does not indicate a prejudice based on class, it is a prejudice nonetheless.  Like the bigoted people he knows who judge others by the kind of suitcases they carry, Holden too "can...get to hate somebody" just because "they have cheap suitcases with them" (Chapter 15).

In J.D. Salinger's novel of teen angst, Holden Caulfield spends several days in New York before returning home to the reality of his having been expelled from school. While he is at Grand Central Station, a place that mirrors Holden's own impermanence, he watches people traverse the station with their baggage, and he is reminded of his roommate who did not have suitcases as fine as Holden, but hid his hoping that others would believe Holden's to be his, and, thus, elevate himself socially in their perception. 

Obviously, then, suitcases are perceived by people as an indication of one's socio-economic class. For, Holden narrates,

The thing is, it's really hard to be roommates with people if your suitcases are much better than theirs---If yours are really good ones and theirs aren't. You think if they're intelligent and all, the other person...won't give a d**** whose suitcases are better, but they do. They really do.

So, while Holden is in Grand Central Station, he views the social strata by means of examining people's suitcases. The two nuns, who have taken the vow of poverty, have rather cheap suitcases with them, but they are unconcerned since they have given up the things of the world. Holden talks with them, but he is slightly ill at ease because he is not a Catholic and worries that they will ask him if he is. He is relieved when they do not ask him, saying his discomfiture is similar to the situation with suitcases: 

It's just like those suitcases I was telling you about, in a way. All I'm saying is that it's no good for a nice conversation. That's all I'm saying.

Holden Caulfield recognizes that there are social and religious barriers among people that cannot be crossed. The suitcases symbolize these unspoken barriers.

What is the point that Holden tries to make about people when he elaborates about the suitcases of the nuns and of his former roommate?

Suitcases are a recurring motif in the book; when Holden meets the nuns, he has just checked his own suitcases into a luggage hold at the station. He feels it's sad that people like the nuns only have cheap suitcases to carry around all their belongings; also he himself has expensive suitcases, Gladstones.

  He recalls his roommate Dick Slagle, who he liked because he's 'intelligent  and... (has).. a good sense of humor' much like Holden himself. Dick also has inexpensive suitcases. Slagle labels Holden's more expensive possesssions 'bourgeois', a difficult word implying disdain for Holden's belongings and his supposed materialism. However, Slagle is secretly jealous of Holden's suitcases because he wants people to think the suitcases are his.

Holden tries to make his point when he says, 'it's really hard to be roommates with people if your suitcases are much better than theirs'. The point is about money and background; Holden can have more expensive suitcases than Dick because he comes from a more wealthy background.  He himself feels that money is not important, and it makes him sad that it gets in the way of human relationships.