It’s easier to tell you why you shouldn’t use house rules. If you play by the book, you can always go back to the book for answers. If players are familiar with the rules in the book, they can assist the DM in rulings by helping to find the relevant rule. A lot of time and energy went into developing the rules listed in the book. You don’t have to worry about game balance because the rules should already be balanced. When new material comes out, it will be designed for the published rules. If you play by those rules you know that the new material will mesh seamlessly with what you already have.
Utilities such as character generators are based on the published rules. Often such utilities have no facility to incorporate any sort of house rules. Character sheets and other gaming aids are designed to work with the published rules. If you use house rules, you may have to create your own utilities and gaming aids.
I hope not! I am actually a huge proponent of using house rules. But it’s important to know the implications of changing the rules and I think everyone should play completely by the rules until they know the rules extremely well. Once you are familiar with the rules, and what will happen if you change them, I think it’s time for you to look at the rules and decide what does and does not work for your particular group.
First off, Wizards of the Cost spends very little effort editing their products and even less in correcting errors, even after they are reported in detail to those whose job it is to fix them. As a result, it is necessary to correct them yourself. Technically, amending a monster’s stat block entry to indicate 160 hit points instead of the listed 60 hit points (due to a typo or printing error) isn’t really a house rule and doesn’t affect much. But that’s the first step towards adopting house rules.
When the first 4e Player’s Handbook (PHB) came out, Gnomes were not listed as a playable race. Many DMs added Gnomes as a new custom race. Not a big change. But enough that players wanting to be Gnomes couldn’t use standard character generators because Gnomes weren’t standard races.
Then when PHB II came out, Gnomes were added as player races. However, this published race may not match the homebrew race many DMs were using. Now you have to decide between using the published version or the homebrew version.
I have seen a great many systems devised to handle spells and spell casting. Of all the systems I have seen used, not one of them matched the system described in any version of the rule books. In other words, not one DM was following the rules where spells were concerned. That’s a pretty major change. It can have pretty far reaching consequences and a number of other rules may have to be modified in order to make the homebrew system work.
That’s the cornerstone of D&D. Each DM makes her world come alive by branding it with her own style and flair and vision of what that world should be. The rules are suggestions and should be altered to fit the specific needs of each individual world.
If you want to play in a generic world with no individuality of its own, go play a video game. D&D is all about imagination and it often takes a great many custom rules to make a world match the vision.