There is a continued and probably eternal debate about control arms (aka "a-arms") and how long they should be for any given application. The upper control arm is one of the things that determines wheel travel, suspension durability, and wheel alignment.
If you're making any sort of suspension modification at all, replacing the upper control arms should be considered.
The most common reason to replace a control arm is to improve wheel travel as part of a suspension lift. However, a new control arm can offer more than just increased ride height (lift) and additional wheel travel:
Many quality suspension kits for off-road vehicles include a control arm that hits all of these notes - more ride height, more travel, more durability, and better quality ball joints.
Additionally, wheel alignment is better compared to vehicles that use some sort of spacer to achieve lift.
This Long Arm Kit features both upper and lower control arm replacements, which allows for a big increase in ride height AND additional ground clearance. See the kit here.
Most aftermarket control arms offer the following pros:
Frankly, a new control arm is almost always an upgrade.
This Jeep has more travel and more ride height, without sacrificing any performance on or off the road. See this page for more info.
If there's a reason to avoid replacing your factory control arms, it's cost. Aftermarket control arms cost several hundred dollars, and for some vehicle owners, that sort of expense just isn't in the budget.
Having said this, the cost of a replacement control arm must be compared to the cost of installing some sort of spacer lift kit. While spacer lift kits have their place, most of them do not offer much in the way of performance improvements. Typically, a spacer lift is just about ride height.
If you were to opt for a very inexpensive spacer lift kit, you can have problems. Specifically:
Over the long-term - and depending on your typical use - a "cheap" spacer lift kit might cost more than a quality control arm replacement.
Or, maybe not. It depends a lot on how often you go off-road, and what type of off-road driving you do.
If you're adding a lift kit to your vehicle, remember that the upper control arm influences wheel travel, suspension durability, and wheel alignment. If you're looking for a suspension modification that improves overall vehicle performance with no compromises, a replacement control arm is a good place to start.