I hope I'm not too late to add.
So in most law firms in countries that follow the American and British system of partnerships, a partner must buy a stake into the firm. Now that they have some skin in the game, if the firm fails then they lose money as well. Partners don't really receive, they get paid via equity from the firm. Essentially instead of paying a high tax that comes with a salary, they pay Capital Gains tax that comes with equity and that tax is very low. However the if one partner is doing illegal activities then in the eyes of the court it will be assumed that ALL the partners had some level of responsibility. As partners you need to hold each other to a high standard so only people you trust, people that you know will tow the line will be made partner.
In Boston Legal it shows that Alan doesn't always tow the line and that he is a loose canon.
In The Practice (The final two seasons can be seen as a prequel to Boston Legal), Alan doesn't confirm nor deny that he was embezzling money from one of his clients. He pretended to be an airline executive to help prevent a young lawyer from being disbarred.
Also in The Practice we see that Alan realises that there is more to life than making partner and having lots of money (check the episode where he helps a homeless dad and his daughter), he realises that his true passion is to fight for those that can't fight for themselves and if that means breaking the law and losing the trust of your colleagues then so be it.