What factors in to how the trombones are notated?

I've seen quite a bit of a variety when it comes to notating the trombones in the orchestra. I have seen a single trombone staff, each trombone getting it's own staff and a 2 + 1 staff arrangement(often with the Tuba also on the staff with 1 trombone). And that's without taking into consideration the Alto Trombone as a solo. Examples:

Individual Trombone staves: 

2 + 1 staff arrangement of Trombones(without Tuba):

2 + 1 staff arrangement of Trombones(with Tuba):

Single Trombone staff:

So, what factors in to how the trombones are notated besides the orchestra size?

The 3 different trombones(alto, tenor, and bass) get their own staff because the ranges of the instrument are different.

Multiple trombones of the same type can be compressed onto one staff for easier reading.

Generally, Trombones 1 & 2 are  compressed into one staff, and 3 & 4 are compressed into another staff.

Solo trombones get their own staff...

It's really simple.

If you're writing for 2 Tenors and 1 Bass, then it's best to have the Tenors together on 1 staff and the Bass separate. Tuba is often put with the Bass Trombone, but that doesn't work very well because it implies the instruments are similar (they're not).

In a concert band score with many trombones, they are usually all separate.