Erich's not entirely wrong rant aside, there are some valid reasons to carry a metal pen.
I carry a pen in my back pocket and the only pens I haven't eventually broken doing that have been so called "tactical" pens. I've carried regular aluminum pens and still managed to break either the ball (very messy) or managed to break the retraction mechanism.
I carry one of a couple of aluminum pens that I own.
The first is a pen made by CRKT that I frankly carry because it is just a really nice pen that feels good in the hand, with a cap to protect the ball and very solid construction that you won't break without using tools or a cutting torch.
It also looks stylish and non threatening. It never looked out of place with a suit and tie or in any conference room I ever took it into. I've also taken it on aircraft, and into county, state and federal buildings all over the country and no one has ever batted an eye or told me I could not take it in, unless pens in general were prohibited items.
It was designed by William Vass in North Carolina and it's only weak point in a TSA discussion is that CRKT added the word "defense" and started calling it the "Williams Defense Pen". More on why that might matter later…
The second is a pen made by Benchmade. It's expensive but very well made and it does have a very sharp carbide tip intended as a glass breaker, that also does double duty as a scribe. In contrast to CRKT, Benchmade got smart and now just calls it their "1100 series pen" instead of the "1100 series tactical pen", although that latter term still shows up in the web address file structure (they need to update that). Again, more on why that might matter later.
Do you need a carbide glass breaker? No more than you probably need a handgun or a parachute, but like both of those items if you ever need one it's very handy to have. For example, if you had to extricate a kid from a hot car, or if you live in a jurisdiction where it is legal to break a window to remove a pet left in a hot car, it's a quick, easy and safe way to break a side window on a car.
I regard both of these pens as practical tools, that are hard to break. I don't regard them as offensive of even defensive weapons, although that could be a tertiary use, as is the case with any metal pen, like a Cross or Parker pen.
Now…let's talk about traveling with them and in particular with TSA.
Despite the "defense" in the name (which I wish CRKT would remove), the pen isn't really any more dangerous or offensive than a metal Cross or Parker pen.
TSA however has fairly recently come out and stated that tactical pens are not allowed in carry on baggage as they are designed as weapons.
I seems to be bit inconsistent on the face of it, as knitting needles are sharper and are legal to carry on a plane at any length, while a Philips screw driver is much more sturdy and can be driven with much more force than a "tactical" pen, but like other tools it is legal to carry on the plane provided it is no more than 7" in length.
The difference, from TSAs perspective, isn't in the tool, but rather in the intent of the design and be extension the intent of the person carrying it. TSA isn't worried about little old ladies with knitting needles, or even big burly auto mechanics with screw drivers. They are however concerned about tactical wannabe types who feel the need to carry a "tactical" pen.
Those same folks also seem to like carrying "tactical" flash lights with things like "DNA collectors" on them. Eventually they'll manage to get "tactical" flash lights banned as well just as they did "tactical" pens.
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So where does this leave us with things like a CRKT aluminum pen or a Benchmade 11 series aluminum pen?
A lot of it comes down to intent. If you are carrying it as a tactical pen for self defense it's prohibited. Period.
For example, if you submit a picture of your regular Cross or Parker pen to TSA and then get stupid and ask if you can carry your Parker "tactical" pen on board, the use of the word "tactical"will get you an automatic "no" response. If you intend to carry it as a weapon, it's a weapon. Period.
Conversely, if you ask TSA if you can carry your pen on a plane and submit a picture, you are far more likely to get a "yes" if it doesn't look like a tactical pen, or say "Smith and Wesson", etc.
Presentation also matters. The CRKT pen is not as likely to set off TSA screener alarm bells, but carrying it with the cap off and on the other end makes it look even less innocuous. Since the final authority is the person doing the screening, how it looks matters.
The Benchmade pen looks less threatening as well with the carbide tip covered, but it still has that knurled grip, blood grooved tactical pen styling that can easily get it classified as a "tactical" pen in the eyes of the screener, regardless of what Benchmade calls it.
It isn't entirely fair but you will also have better luck getting either of them through TSA if you are dressed like a non threatening human. If you insist on traveling in tactical pocketed pants and a "punisher" logo tee shirt your odds go way down. Add a tactical ball cap and a wannabe operator beard and you are almost certainly not getting on the plane with that pen in your carryon.