AGM batteries do perform better under more demanding duty cycles, like start/stop applications, as well as having lower self-discharge rates, and better durability, at least in theory.
They also have their own particular charging needs, and like to be kept at higher states of charge than FLA batteries, or their durability may suffer. They rely on the recombination of the gases produced during charging (which are normally vented to the atmosphere in a FLA battery) to maintain capacity.
While designed to operate as a sealed, non-venting unit under normal use, they do still contain a relief valve to allow venting under abnormal conditions, like if overcharged. So for the cases that do have provisions for venting, when used in interior mount applications, it is still a good idea to install the vent tube in case that does occur. Those expelled gases represent lost capacity, and can't be replenished, like topping off an old, unsealed FLA battery with distilled water.
In practice, installing an AGM isn't going to cause your truck to blow up, and probably won't make any significant difference in operation. But "not had any problems" doesn't mean optimal operating conditions for an AGM.
So to compensate for the charging systems, or driving patterns, that don't charge AGMs at higher levels, some owners keep them on maintainers, or do a periodic full charge with an outboard charger (preferable with an AGM setting).
The question then becomes why pay the price premium for an AGM battery, when their advantages aren't required, realized, as well at risk of being in jeopardy because the charging system isn't fully compatible?
The answer to that reaches beyond the technical aspects, into the fact that AGM batteries are marketed as upgrades, or "premium" products, without any regard or consideration of their application. Many buyers, especially those with some urgency in need, probably just want to buy the "best" battery available, with a "good" warranty, at the highest price they're willing to pay. If it starts the car, doesn't fail prematurely, and most importantly among neophytes, simply fits, they can remain blissfully unaware.
AGMs, and their cheaper equivalents, EFBs, are now firmly in the mainstream, so for applications where they come as OE, the only wise choice is to use the proper type.
Knowing the situation with older applications, it's still a judgement call, and up to you.
A cheap, instant read conductance tester will only present a narrow view of how a battery performs. They measure a couple aspects of a battery, and rely on assumptions to rate other aspects. The only true way to grade a battery is to fully charge it, and discharge it under load to emulate a full cycle.
A person can be obviously ill, but not run a fever. Body temperature does serve as an indication, but shouldn't be taken as the sole arbiter of the person's condition. Same with those testers, and I suggest reading up on them to fully understand their capabilities, and limitations.
