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.
Why Replace A Control Arm?
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:
Increased durability in the arm itself
A wider range of uniball/ball joint options
More compatibility with replacement coilovers
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.
Control Arm Replacement - The Pros
Most aftermarket control arms offer the following pros:
Stronger than stock - While some vehicles have better quality OEM control arms than others, very few offroad vehicles come with an OEM control arm that's as strong as a quality aftermarket unit. Strength is important whether you want to go fast or crawl over obstacles.
Upgraded ball joint - A uniball that's ready for off-road is not usually compatible with a factory control arm. Generally, a factory ball joint is all that will work with a factory control arm. That's very limiting.
Improved ride quality (depending on shocks) - There's absolutely no reason that an upgraded control arm has to reduce vehicle ride quality. In fact, most coilover kits for off-road use offer a smother, better ride than factory during regular day-to-day driving. Aftermarket control arms often weigh less than OEM control arms, reducing unsprung suspension weight and improving ride quality. Aftermarket control arms can also accommodate replacement shocks that offer better all-around performance than OEM shocks.
Increased suspension travel - Almost all control arms allow for a larger coilover kit to be installed in place of the OEM kit. This can make for several inches of increased travel.
Increased ride height (if that's what you're looking for) - Last but not least, a new control arm almost always gives the vehicle owner the option of increasing ride height.
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.
Control Arm Replacement - The Cons
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:
Spacer lifts can reduce ride quality
Spacer lifts can increase wear on factory ball joints
Spacer lifts can cause serious suspension damage (leading to major vehicle damage) if fully compressed while off-road
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.
Summary
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.