Fortunately, since federal law governs express written warranties, the FTC says there are certain inalienable consumer rights that cannot be disclaimed by manufacturers or retailers. An important one to note is the specific prohibition of "tie-in sales provisions" in which part or all of a warranty is denied because the consumer didn't use accessories, service, or even parts from the manufacturer. You cannot be limited to using something that will bring more sales to the brand if a suitable substitute is used. Remember, this even includes parts and service. You can take your tool to anyone for service or even do repair work yourself without affecting your warranty coverage under federal law as long as you don't screw it up. Manufacturers are not required to cover under warranty any problems caused by faulty workmanship or material by someone else. Certain parts such as specific switches and accessory batteries can be required by manufacturers legally, but replacing the correct brushes or a suitable power cord should be fine.
Manufacturers also cannot actually preclude or limit implied warranties of their product. This is a common violation found in writing on many warranties, and even if it is presented with a suitable-looking disclaimer, all states provide for implied warranties and you are covered regardless of the verbiage on the express written warranty of the manufacturer.
One of the most important things to remember is to keep proof and date of purchase along with the most comprehensive wording of the warranty available at the time of your purchase. The version written in your owner's manual is not always complete. Don't rely on a customer service person to know what the warranty coverage was five years ago. Claims cannot be pursued in some cases without having all the documentation. Some manufacturers will just go by the date code on the tool or by the filing date of a mail-in card if it was sent, but not all.
When taking a tool to a service center, keep in mind that they make a lot of "warranted" repairs based on their own judgment. Being polite, cleaning up the tool, and presenting it in a clean case can mean a lot more than what year a card was mailed in. Many repairs are made and not charged as a goodwill measure by service centers but be prepared to follow what you have in writing.
In our 2006 Tools of the Trade reader survey, professional tool users like you rated the importance warranties have in their purchasing decisions behind quality, durability, precision, performance features, ease of use, brand reputation, versatility, availability, price, ease of maintenance, and service–in that order.
So whether this trend is seen as a marketing ploy or not, tool companies are clearly confident enough in their tools to expand their product coverage, which is great news for all of us.