I dont think theyre always in bad taste, just often so.
I found an old shirt that was made for me with my initials on the lower left, just above the belt line. I think I didnt like that the initials werent obviously handmade. I used to get these small, obviously handmade ones but even that was an expensive waste of time. And what if the monogram gets botched?
There is such a wide diversity of reactions to monograms that it is hard to say whether it's worthwhile or not. I have seen whole groups favorably impressed with a man's monogram on the collar or cuff. That placement almost always makes my lip curl. The only exception Ive seen was this client of my shirtmaker who had two sets of buttonholes on each of his french cuffs with a different major city around the world (NYC, Tokyo, London, LA, I think). He wore tiffany watch cufflinks in there and apparently he worked in a trade where he needed to know differnt timezones. that's so unique that what mightve been crass becomes appealing.
Be prepared to have your privacy invaded constantly because we are in a time where people will ask you constantly what the monograms mean. In an age of legible clothing, the expectation is that someone will ask for a translation. If that's what you want, then fine. But let's say youre in a situation where you dont want someone aksing what the initials stand for?
For me personally, I recognize that the way I dress already garners enough attention (suspicion?) that loading on the bells and whistles isnt a good idea.