It's a tried and true practice that many carriers worldwide employ. To fight with the big guns, though, which are rolling out LTE (Verizon) and HSPA+/ LTE (AT&T) like there is no tomorrow, one has to differentiate with value, right?
After the rumors that T-Mobile is lowering the prices of its Even More plans, now the carrier hints at changing or removing that 5GB soft data cap in a training leaflet for its reps too. It states that "unlimited plan for more than 5GB monthly data use will be launched sometime this year".
Make of it what you will, but it seems that T-Mo is either raising the cap (more likely), or removing it altogether.