Cross-Class skills are limited in two ways: Their maximum rank is half that of cross-class skills, and two skill points need to be put into the skil to increase it by one rank.
This is done for niche protection. In this way, each class has its skill that it is good in, and doesn't encroach on the "territory" of the others. In this way, it is unlikely that the fighter of the group will be the chief diplomat of the group, or the rogue the expert on the fine points of theology.
However, I think this is just a bit too restrictive. This all but prevents characters from developing side skills in which they might not be first rate, but in which they are at the very least decent. Why shouldn't a fighter be able to be halfway decent at Sense Motive, especially since that skill can also be important in combat? Or why shouldn't any other character dabble in some purely acedemic knowledge skills?
Therefore I would suggest keeping the maximum ranks for cross-class skills, but making them cost the same number of skill points as regular skills. This way, the niches of the classes remain protected when it comes to skills, but it does allow for some more diversity among characters. With the double cost, characters will be hard pressed to even catch up to other party members who only have high ability scores linked to the relevant skills, but without this restriction, they can at least be somewhat credible in these skills.
What are your thoughts on this?