If lows are the bones (the structure of a tone), low-mids are the flesh and give Bass its body. Many bassists, specially in metal, take off all mids to open space to vocals and guitars. This can be effective in bigger sound systems, but smaller cabinets simply can't reproduce frequencies below 65Hz. So, you suck all life out of the Bass if you doesn't let any mids to go through. They can help one to feel the missing frequencies, making the Bass being heard more easily.
I solved this issues with a good and loud Bass Amp. Now Bass tone always feels right to me and any issue regarding people hearing me is sound tech issue or the house (if they doesn't are equiped to sell music to public). Now we have transformless class D amps and neodimiun speakers that let a bassist have a powerfull, but tiny and light gear at our disposal. Not as pleasant as gigantic tube amp classic stacks but very effective.
