You start getting into the life factors (like Creator said). If I'm stressed by work, or on the road a lot, or haven't been sleeping well, or the weather is shit then my training will be affected. I'm not a professional athlete who has options of recovery and rest, I have commitments. So as a result training is always a compromise of what is most effective given our daily lives.
I see upper/lower and fullbody as great programs that can be the best training regime but they are also able to fill the role as a compromise for busy people as well. Splits don't have that option as much. So I think any program has to have your lifestyle programmed in, otherwise it won't work.
This is A quality thread, despite the bickering.
Like said, there are far too many variables to say frequency is the most important part of a training regime. Not just stabilizing genetics, caloric expenditure and body composition play roles in training routines that most people forget about. For instance, training my entire musculature 2x/wk, using a complex split strains my neural system. Thus, i'm able to a body split, excluding frequency as a principle. Bottom line: it's the person's decision.
Scientifically speaking, frequency is a component of hypertrophy.
As far as DOMS; it is is not a sign of growth. As far as other variables, it can be a result of:
Change in training principles (sets, reps, exercises)
New fiberal patterns
The CNS plays a huge role here, and is probably the biggest factor to including frequency or to not. Your muscles will recover much faster than you will neurologically, that's a fact. Most people believe soreness is caused within the neuromuscular junction, rather than the sacrolemma.
Either way, protein synthesis is reduced.
just my take on it. As all hypertrophy component threads generally turn to DOMS.
But furthermore, there are a TON of misconceptions related to "lactic acid", depending on the context, stating that as soon as you've got LA could be a bit misleading. For example, did you know that sitting here right now, your testes are using lactate as a substrate for energy? Lactate is something i've taught about in an intro to exercise phys class, I'd be happy to go over it in another thread.
I like more frequent training like ub/lb or fb. from my personal experience it just feels better and it made me grow better as well. I had to get used to it before since I trained split only but now I would not change it back very quick. On the other hand it depends on someones experience level. Imo someone whos an elite level lifter could benefid more from split type training than a novice/intermediate. Ub/lb or fb routine are a bit more tricky to create. Peopel tend to overdo things which is not needed. Simplify and win!