I thought that we established that the ranking system is more accurate than Halo 3's?
The System:
See what they were trying to say is that you probably gain or lose XP through kills, deaths, headshots, assassinations, wins, losses, quitting, etc. Whether you go positive XP or not depends on you. It is likely that you can rank down if you have a negative K/D and you lose, because you go negative XP. Heck, if you die a lot, but still win, you STILL may go negative XP. It depends on YOU. So, yes, a type of 1-50 system is present, but it isn't shown to others, AND it isn't numbered. Plus, it is very accurate. Once you hit the current level cap for your account, you can use a specialization to increase the level cap so you can rank up even farther.
Boosting:
Granted, you can easily have four people make new accounts so you can just kill the crap out of them to gain XP with no ill effects to their real accounts, but I'm sure 343 would take notice of new accounts not earning any XP. Therefore, the only way to boost is to either supress the other team, or to cheat with new accounts, which I'm sure 343 would be watching for.
Spartan Point Boosting:
I would believe that you can't get more SP by deranking to re-earn the rank you just lost. Therefore, the only way to boost SP would be to boost rank, which is not likely to be easy.
Does this console any worries?