Maybe someone can educate me on this. I've always thought guitars needed to be played to make them continue to open up over the years? I'm concerned about this because of a friend's guitar collection he left to his brother after passing away from cancer. His brother told me he might store them for years until they increase in value. He's in humid Houston, TX and as someone pointed out to me in another post this should be a concern. The brother who inherited them doesn't play and I'm too far away to go there and play them. Am I right about them being played?
I don't know that there's any definitive answer on the topic of whether playing (or in this case, not playing) a guitar is of any particular benefit (or detriment) to it's well-being or tone. My personal take on the topic is that [regularly] playing a guitar helps the wood, glue joints, etc. to 'adjust' more quickly from it's old life as wood/glue to its new life as part of a guitar. Over time, that would happen anyway; so the resulting vibrations when playing, I think, just helps to accelerate that process. Some have even suggested that the effects are merely temporary; if so, playing a guitar that's sat for a while will bring it back to life, so to speak. Frankly, I see this effect with my guitars, which often sit for a couple of months or so between playing sessions: they sometimes sound a bit dull at first, then at some point during that particular playing session suddenly "wake up" and sound great again.
I think the main contributor of what is generically referred to as the 'opening up' process is simply age. The wood begins to crystalize and becomes something a little different than the wood that was originally used to make the guitar. No amount of playing will really affect that; it's just going to be time and luck-of-the-draw with how the wood ages, environmental conditions all being the same. Which brings me to the next point...
An answer more-or-less relayed straight from the horse's mouth (i.e., Taylor Customer Service) on the topic of storage environment: humidity control is the single greatest consideration when storing a [Taylor] guitar. Not sure if that's just 'party line' from Taylor, but that's the essence of what I was told today by their CS representative. This position is, again, somewhat supported by my own empirical experience with Willcutt Guitars. Willcutt's has an enormous guitar inventory; they've got some NOS Taylors that are approaching 10-11 years old in their stock (properly stored, of course). And they sound just fine whenever they pull one of them out of storage for me to play.
But the reason I was on the phone today with Taylor is because the HVAC inside my house in Central Texas has decided to go on strike -- if not give up the ghost for good; and since Mr. Murphy is a sadist, he's seen to it that the A/C went down for the count just about the same time that high temperatures in Texas are starting to push the century mark... So I was asking Taylor CS what environmental conditions (both humidity and temperature) signaled the need for an emergency evacuation of my guitars to my neighbor's house -- and it sounds like I'll be OK until I can get an HVAC contractor in there next week -- as long as we keep the humidity from climbing too high inside the house (I'm guessing north of 75%).
It seems like higher humidity is more damaging than higher temperatures; if humidity is kept in the Taylor target zone (45-55%), the guitar can tolerate much higher temperatures than they will likely see inside my house, even without A/C. But if humidity becomes excessively high as well, not only can the woods warp from the moisture, but the glues can start to lose adhesion due to the combination of heat and humidity. Fortunately, I'm not in Houston -- where humidity during the summer is never in short supply...
A lot of rambling to get to this particular point: it's challenging enough for me, while my job is keeping me on the road, to keep a tight rein on the environmental conditions in which my guitars are stored. In the winter, the heating system (all-electric) dried the air out terribly, so humidity control was much more challenging during that time of the year. Your buddy's brother will be contending more with what I'm facing at the moment (heat and high humidity). So if he isn't going to closely monitor and care for the guitars while they're in long-term storage, he's probably doing them greater harm than he would by not playing them.
Given that his stated intent is to keep the guitars solely to speculate on their future value, it doesn't sound to me like he's going to put much (if any) real effort into the proper care of them -- which ironically, will hurt the value in the long run if they become damaged due to poor storage or maintenance practices. In my opinion, if his deceased brother's guitars aren't something he wants to keep for himself -- either for sentimental or practical reasons (like learning how to play) -- he should just sell them now and be done with it. You mentioned that he inherited the guitars -- which means he's [personally] got zero dollars invested in them at the moment. Proper care and feeding of them will require a monetary investment on his part (at a minimum, humidity control packs; possibly a climate-controlled storage locker; etc.). You've already given him good research on their current market value, so anything he makes by selling them right now is pure profit... Seems like his proper move is a no brainer to me...