Why do modern jetliners provide Thrust Reversers at all in their design decisions?
There are  a couple of reason for the presence of thrust reversers:

To reduce wear on the brakes.

To provide a margin of safety (for example in wet conditions).


In 1995, NASA carried out a survey on airlines Why Do Airlines Want and Use Thrust Reversers?. The results of the survey, though dated, give an indication of presence of thrust reversers.

The main reasons given by the airlines was to provide additional stopping force in adverse weather and to reduce brake wear.


The first question posed to the airlines was &quot;why do we have thrust reversers?&quot; (...) The number one reason given by the airlines was to provide additional stopping forces in adverse weather conditions (i.e. on wet, slushy or slippery runways). The airlines also use thrust reversers to reduce brake wear and to provide directional control and additional safety margins during an aborted takeoff (RTO).


According to the airlines, the thrust reversers were there because they wanted them there. This meshes with the fact that the thrust reversers are not used in calculating stopping distances during certification.


When the airlines were asked who they perceived as the principal organization behind the installation of thrust reversers on commercial transport aircraft, the airlines responded that they were. That is, the
aircraft manufacturers put the systems on board the airplanes because the
airlines want them.


Most airlines recommend using the thrust reversers early in the landing to minimize braking.

The airlines weigh the economic benefits of using the thrust reversers (and engine wear) against the use of brakes and the results seem to indicate a significant amount of money saved by using thrust reversers. The report notes:



... the savings in brake system costs due to the use of thrust reversers was estimated. The results show that the total yearly savings in brake system costs is about \$8.6 million, or an average of about $17,700 per airplane for airline 17.

Though this is almost certainly an exaggeration, the numbers indicate that the airlines would definitely prefer thrust reversers over increased braking.
In the end, the reverse thrust can be important in reducing the runway requirements in case of a wet runway. In 1999, a Qantas 747 overran the (wet) runway in Bangkok and landed up in a golf course. Qantas has used only idle reverse thrust rather than full power during the landing as per the operational procedure. The official accident report included data from Boeing 747-400 Flight Crew Training Manual, which stated that,

Immediate initiation of reverse thrust at main gear touchdown and full reverse thrust reduce brake usage, thus minimising brake temperatures.
To minimise wear of carbon brakes, minimise the number of brake applications. Use maximum reverse thrust and operate the autobrakes normally.
The importance of establishing the desired reverse thrust level as soon as possible after touchdown to minimise brake temperatures, tyre and brake wear, and to reduce stopping distance on very slippery runways cannot be overemphasised..

As a result, Qantas revised their operational procedures, requiring full reverse thrust. Brake wear is an important reason for having thrust reversers. For example, in Airbus A380:

... unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.

The BAe 146 lacks a thrust reverser; it makes do with an airbrake in the tail and spoilers all along the wing for braking (which should give an indication of how much spoilers is required to eliminate reversers). The 146 got away with that because of the low touchdown speeds (~90 kt) and the huge airbrake, which is not avialble in the other commercial airliners. For the same amount of spoiler area in other commercial airliners (in absence of airbrakes), they (spoilers) would occupy most of the wing surface, increasing weight and complexity. Some Embraer ERJ 145s also lack thrust reversers (they are optional).