Football is considered the profession of footballers, the place where they can earn money. It is normal for them to work on improving their performances to sign better contracts, I can't blame them for that. This isn't exclusive to football, but it is the ambition of every sane human, whatever his profession is. I know he's done his best in all these clubs, I've been following him since his early days of his career. But all of this is a thing and loyalty is another different thing. Crespo for example could have played for just Lazio or Inter, earning the salary that he wanted.
I can't call this loyalty, it is called the lure of the green pastures.
The problem is with his clubs. It wasn't Crespo who was leaving his clubs but his clubs were selling him to get money from him.
Parma had big financial problems when they sold him to Lazio, for a world record fee of 34£million to Lazio. If you remember , the next year Parma also sold the rest of their stars Buffon and Thuram, and then even Cannavaro.
Then Lazio had financial problems and had to sell him to Inter for 17£million. Lazio lost half of the money they payed to Parma 2 years ago, but they were forced to sell their best player.
Crespo never wanted to leave Inter in the first place but Moratti decided to get the money back he payed for him, and he sold him to Chelsea for 17£million.
In Chelsea and England he wasn't happy at all and this is the only case where you can call him not loyal. He was the one who wanted to go out of England and that's why Chelsea loaned him to Milan. He begged Berlusconi to buy him from Chelsea, but Berlusconi didn't wanted and he bought Gilardino. Hernan Crespo, like every proffessional accepted that and went back to England.
Chelsea loaned him again to Inter, and now, although he still wants to stay with Inter, Chelsea will probably get him back and Moratti won't care about Crespo's wishes and won't buy him from Chelsea.
Maybe he wasn't the one who wasn't loyal to his clubs, but his clubs weren't loyal to him ?