Vincenzo,
I believe the complaint for keeping limit separate from no limit is primarily because of the spread limit games and not the structured limits. Spread limit allows any wager, within the spread, on any street. $1 to $3...$1 to $5...$2 to $10, etc. Fixed bet, or structured limit ($2 & $4, $3 & $6, $10 & $20 etc), is the easiest to understand, at least that has been my experience through many years of teaching. When we consider that the minimum bet of $1 in a $1 to $3 stud game can be initiated by the original bettor on any street, even seventh street. This is where it gets tricky...let's say Player A bets $1, the options to the next active player are simple enough, he may fold call the $1 or raise $1 $2 or $3. If he raises $1 the cap on that round could total $4 if the next two players raise $1 each. This of course applies because of the three raise limit in most rooms. This can not happen in fixed bet limit poker. The spread limit games are unique in that respect. Betting $2 in a $2 to $10 spread limit and having the next player go all-in for $3 qualifies as a raise when using the 50% rule...it will also alter the cap.
Bottom line...fixed bet, or structured limit, (witch ever you prefer) is the best because it is so much easier to understand. At least, that's how I see it.