Hiya,
I think there is room for both modem-only and tether-only options, but whether GliNET want or need to go the former (more tech-heavy) route is down to GLiNET’s business model for each router / device I guess. There are, as alzhao has pointed out before, options in the form of ROOTer firmware (OpenWRT).
For the record, whilst I am not as tech-savvy as most here, I am reasonably good at following directions so after many visits to various (and often dodgy-looking) russian modem firmware sites I did revert an E3372H to stock mode using the ‘needle method’; it was not altogether that difficult (but having said that I killed my E8372H albeit for a different reason) to beware 
See also some input here on the same theme, I provided two E3372Hs, one in ‘Stick’ mode’ (flashed as above) the other in original HiLink ‘Tether’ mode. I will be interesting to see if GLiNET can advise any discernable real-word difference in speed. I agree with alzhao though, most (90%-95%?) of purchasers just want to plug and play any modem (or indeed phone) they may use to run their router.
cheers
k.