SLASO will not only challenge your skills as a Halo player, it will also test your patience, willpower, and ability to find ways around obstacles that would leave the common fps player furious. The pros to pushing yourself through these challenges may be subtle to you, but to everyone else you will be the badass that tops the multiplayer scoreboards. And you will be the player that others in matchmaking come to beware, or perhaps even aspire too.
The skill benefits are nothing to sneeze at, yet the (rather addicting) new credit system introduced to Halo Reach, as rewarding as it is, is also remarkably flawed. The credits that you would earn for playing a campaign level on normal is no different than that of playing a level with SLASO. A rather poor compensation for any time or effort invested in a grueling level.
S.L.A.S.O. or "Solo Legendary All Skulls On" will surely be an arduous test of your skills against the formidable, & unforgiving convenant horde. SLASO as the acronym implies, pits you (alone) against heavily beefed up enemies, enemies that are already enhanced by the legendary difficulty setting. The covenant you will face in SLASO will be unlike that of any you have squared off against in the "average" campaign experience.
The multiplayer matchmaking of Halo Reach may be a good place to start in terms of getting credits and getting better at the game. However, I've learned from experience that MM over time simply leads to frustration and headache, especially when it comes to "teabagging". Although the poor, short term benefits of SLASO discourage the attempt of it at all, the long term skill improvements are worth the time spent to hone your halo prowess, better to know your stuff before jumping into "monkey pen". You'll be shooting through the ranks of Halo Reach far more skillfully, and efficiently than by stumbling around Matchmaking for hours on end.