Particularly being able to adapt illustration styles and techniques will open you up to more work. Just sticking to one theme, even if it has been lovingly developed by yourself will restrict your workload and money you earn...we all have to eat after all.
Having more strings to your bow will also make life a more interesting, as they say variety is the spice of life!
As an Art Director I have commissioned many different styles of illustration never using the same one, unless it was part of a campaign.
This kept my portfolio of work looking diverse too. So adapt, be a chameleon and you may make a few squid!