I use my 6x24 the most - I hear you about all these special rulers and find myself staying away from the patterns that require them unless I really fall in love with the pattern. I think the ease of use is one reason some buy every ruler- but if you have the angles and sizes covered by another ruler just use it
Like making a 5 inch square - that could be made by using the 6x24 ruler but would be a lot easier if you had a 5 inch ruler. So if you can make do with what you have go for it