In the original play and non-musical Russell film (and probably even more so in Patrick Dennis' novel) Beau is very unabashedly and unapologetically a plot device and that's part of where the humor lies.
There was really no attempt to make him into some revered hero/leading man type. The stage musical but especially the Lucy film strive to give Beau a sentimental significance that Patrick Dennis didn't worry about. The "love affair" is between Mame and Patrick.
Perhaps by the 1970s building up Beau's character seemed essential for some reason (and Robert Preston's casting enforced it). The source material is more of a lark and IMHO got more treacly and less anarchic with every succeeding version.