THE SCOOP:
The ENGINE is NOT a BRAKE!
That having been said.... you CAN brake WITH the engine.
Engine braking has its place and time. It serves a purpose in generating resistance through the motor causing "additional drag" on the driven wheels of the car. This can help maintain control in downhill and other situations where off-throttle deceleration would otherwise result in a "near-coast" condition with an automatic... (similarly low friction with a manual... although not as much) As mentioned before, rpm matching is important for SMOOTH and UN-ABUSIVE downshifts. JERKING is not effective nor good for the car... nor is HARSH TRANSITION down into a lower gear. Lash places various stresses on clutch packs (A/T) and drivetrain components... and exposes them to SHOCK FORCES that are "inconsiderate" of their durability. In a BRAKE LOSS situation, or a RAPID BRAKING situation, the use of the engine as a "supplement" to the brakes can aid in deceleration, but use of engine braking ALL THE TIME, depending on the severity of use, WILL deteriorate the performance of the tranny more rapidly than normal or moderate engine braking wear.
I do it, but not as the primary source of deceleration... that would be imprudent, IMHO.
Brake pads are CHEAP by comparison, and DESIGNED to be the primary point of wear... a consumable... I don't think you want to replace trannys at the same interval as air filters or tires....
(Coming from a guy who effectively WELDED a Supra rear end pig into a "static axle" by running 4:88 gears and engine-brake-down-shifting hard enough to send back-lash the magnitude of a tsunami through the planetaries, clutch, and drivetrain until universal yokes were twisted and diff and tranny had to be replaced...)
Price of replacement "pig" = $275
Price of used tranny = $600
RUBBER BARK and WHEEL HOP when decelerating = PRICELESS
-crisp
