The Modern Business Approach

Disk space is very cheap (eg, Google Mail provides about 8GB = 8 x 10^9 half-width characters for free). Trimming is no longer necessary for infrastructure cost reasons.
Business communications are often simple decisions: "Revise your document." "Accept the vendor's offer." "Come to my office." I.e., detailed discussion is more often conducted in meetings (one-on-one, or all-staff). Point-by-point discussion in email is less necessary, and often moved to an attached wordprocessor document.
Business communications involve people "all on the same page". They have common knowledge of the business ("same joshiki"), and generally already know the context. The quoted material is like an appendix that most readers don't ever look at.
These factors lead to the top-posting style.