ok..........
let's dive in.
bloodstone, you won't have problems unless you have out of phase information. phase affects low end tremendously as the wave forms are much longer and a slight shift in phase will affect certain notes and not others. the degree of the shift is what is important.
when mic'ing a bass and and taking the DI the DI signal will arrive sooner to the multi-track then the mic. see the mic has to pick up the acoustical sound through the amp circuit into the speaker then to the mic, through the cable to the pre-amp and then to the multi-track. it just takes longer to follow that path then the DI
this is when phase is an issue. if you hard pan it with out any timing correction you will have some phase cancelations when summed to mono. this is basic engineering so i won't beat it to death.
radio......what happens here is that when your stereo hits weak spots in the transmission it will automatically switch to mono. this is a summing and not a ditching of the left channel. it sums the feed and pumps it out. this is when good mixers rise above the rest. there work won't sound any "different" in mono...just less full...
vinyl records have trouble with stereo bass do to physical limitations of the medium. i believe most lathes some the low frequency (like below 150) to mono and cut it in that way. i'm not a vinyl expert nor do i care to discuss it past how much i enjoy listening to my records.
old beatles recordings:
the beatles recording at EMI studios, which happened to be fitted with an EMI console (go figure). the EMI consoles had stereo channels only with hard panning. so really, the engineers had no choice. bass went to one channel and drums went to another. it was there best option. it split the rhythm section. drums left, bass right. that's why the lead vocal is left and the slap is right.
in this situation, summing to mono will merely bring all elements front and center. what Liam is talking about is some other issues on more adventerous beatles' recording where things really are out of phase. i've heard some of those as well.
so, when i say i won't adopt that practice, i hope i've clearly stated why.
little labs makes a brilliant piece of gear called that has variable pahse control to re-align a DI with a mic and eleminate this problem. i suggest you look into picking one up. then again, if things are working for you.....don't change a thing.