Rules. As an RPG player (which includes GMs) rules are as vital as air or water and as damnable as the worst pests. Rare is the player who doesn't have a rule they dislike in even their most favorite game, and that often results in house rules. What are the pros and cons of house rules? Is it all sunshine and roses, or are we losing something in the process?
Pro: Diff'rent Strokes
Maybe it's the fact that we all have different tastes, or maybe it's due to the fact that RPGs are games that encourage creative solutions. Regardless players and GMs alike often find cause to create house rules for their games to suit their taste. If you think that rule for stun is too punishing and can really make combat un-fun you can tweak the stun condition to be less severe, or maybe tweak the rules around it so that a stun is harder to "earn."
This is house ruling for personal taste, and it is a huge pro in favor of house rules. RPGs are a game to be enjoyed by everybody at the table after all, and what better reason to change rules than to help ensure everybody has fun?For a hobby about having fun, being able to control your enjoyment (and that of your fellow players) is pretty important. Playing with an RPG you customized also means you probably can work the rules into play more seamlessly.
Con: Losing the Intent
To loosely paraphrase Jurassic Park, "Just because you can make a house rule doesn't mean you should." Games are designed by people. People who have to choose how the rules inform and influence the gameplay. An RPG where social interaction is prized above violence will have rules to support those interactions and downplay violence (or possibly to make violence so hazardous that avoiding it is as wise in game as it is in real life). Understanding the designer's intent can be helpful for understanding the game's tone and feel and how the rules inform that.
It's important to understand how a game's rules as designed influence how it plays. Look at the way combat lethality can impact the length of encounters, look at the way higher level play impacts the capability of the players to impact the world around them. If you don't pay attention to the design intent when you house rule you may find the feel of the game changing.
Pro: Experience by Design
Picking apart and rebuilding rules gives you experience with how those rules work, how they are built and how to build new rules. For GMs this helps with encounter design, by helping us understand all the moving parts involved in a game. For others house rules can sometimes even lead to people designing their own games. Heck, the hobby exists because Gygax house ruled his historical miniatures games into an entirely new genre! That's not to say that a few house rules makes you a game designer. There's more to game design that tweaking what has come before, and innovation often requires a blank slate.
Con: Losing Sight of the Past
Where Gygax had to build the rules that would define a hobby from nothing today we are anything but starved for choice. There's so many RPGs out there that it seems like you can probably find one that fits your taste perfectly, but that hardly stops us from tweaking and changing, fiddling, and creating our own new rules. I've even seen discussion where people talk about house rules without even having played the game as is.
Forty years later we have an entire genre of RPGs that are looking back at the games of the past. The OSR game community is looking at what we've evolved away from and finding that while we may have come a long way in the right direction in some respects (ascending armor class anyone?) but they are also finding that we've lost something along the way. Sometimes new rules help a game, or all games, evolve, but sometimes you lose something that you'll miss one day.
Pro: Dial "G" for Gritty
What about when you like the mechanics of a game system, but want to change the tone? I mentioned above that rules inform the gameplay. Sometimes the intent behind a house rule is to change that gameplay to change the tone of a game. Not all superhero games are the same for instance. What works for the high power "Silver Age" of comics doesn't fit the dark and brooding "Iron Age" of the late 80s. House rules to change the tone often involve minor tweaks to character capability and/or the combat experience (let's face it, combat/conflict rules are like 90% of most RPG systems). Being able to dial the tone
Con: Horses Don't Push Carts Very Well
The urge to create house rules can come quick for some. Quick enough that I've seen people discussing changing the rules to a game before they'd even played it once as written. Maybe this is more of an opinion than a hard and fast rule, but I think you can jump the gun on house rules. It's important to play the rules as written before you start changing things. This helps you to understand all the interlocking parts. RPGs are complex things and if you intend to start modifying parts of them it stands to reason you will have a solid understanding how the work. I'm not saying that it's impossible to predict how the rules will play without playing them, but I do think there is value in giving a game its due before taking a scalpel to it.
All of this brings me to the question: is there value in the house rules? Yes, of course. The catch is that you need to be aware of everything that a house rule will do to your games. There's certainly a lot to be gained, and maybe even more to be gained than lost, but all of this requires some thought and understanding.