Ti E-series heads? Intriguing. Aleph-compatible?
I prefer battery tubes with non-removable tailcaps, because removable tailcaps don't work with McGizmo-style deep-carry pocket clips. That's why I went with McGizmo 2x123 Clicky Paks for the Ti lights I assembled last year. With Aleph-compatible E-series heads, it's even easier to change the batteries by removing the head, because the light engine stays in the head and doesn't fall out. That's why I decided to use your E2C adaptors in conjunction with C-series heads, because it gave me the flexibility of using P60-compatible light engines while allowing me to treat the head as a single unit that could be easily removed, like an oversized E-series head.
Obviously different people have different tastes, but given the reasoning I just outlined, I think a setup with an E-series battery tube with a non-removable tailcap (possibly overbored to allow 17670 cells to fit comfortably) offers the best flexibility. The user can use an E-series head for maximum sleekness, or a C-series head with an E2C adaptor for high-power applications without the clumsiness of having a loose light engine waiting to get dropped on the ground. As long as the battery tube uses a McGizmo switch, there won't be many people who will need anything better -- though I suppose I can see certain circumstances where a solid-state switch like a FETtie might be useful, but a FETtie could just as easily screw in from the outside, so there still wouldn't be a need for a removable tailcap.
TL;DR: removable tailcaps are useful for two things (as far as I know): changing batteries without dropping the bulb on the ground, and installing a better switch than the light came with from the factory. Both of those problems can be easily avoided using parts you already have available, without needing a removable tailcap at all.
