"most people" != all people.
It's not a gimmick because not everybody uses a particular phone or line every day. When I was still on an AT&T prepaid feature phone, I hardly ever used up my voice minutes and might use the phone 5 days or so in any given month. When I activated that line, I started on a pay-per-day plan, but switched to a $25/90 days plan when that became available.
If you read the Howard Forums, there are also plenty of power users there who swap SIM cards or use multi-SIM phones. They might use the T-Mobile service when their voice minutes run low on a grandfathered unlimited data plan, or if they travel overseas and want to take advantage of T-Mobile's international roaming. A lot of them are absolutely livid that T-Mobile has eliminated the $3 unlimited daily voice plan.
Also, what you considered bloated bundles, are actually cost savings compared to what the a la carte pricing could be for typical users. Compared to the T-Mobile $30 plan, what unbundled cost structure would provide savings for consumers? How much would consumers pay per minute? Per MB? Would stuff like tethering also go to a per use fee? Would music streaming and international roaming also go by the wayside and revert to a fee per use basis?
Sure, most consumers never use up all their data and they know it. But, they also want to have it available when they need it. I don't think people are as clueless as you seem to think. It's the same reason why so many people pay more so they can get AT&T or Verizon's network. They might live and work in areas with strong T-Mobile coverage. But, they will pay more for those occasions where they might travel into outlying areas that T-Mobile's network does not reach.