Having a credit line can help when it comes to getting comped. The casino knows you're "good" for that certain level of money and they'll do everything they can to take it from you - but in return, they'll also be generous with the comps because they know you'll expect that... This has worked well for me numerous times when on vacation at Las Vegas properties, for example.
Betting on credit is NEVER a good idea - but it's fine to take a Credit Line, which is essentially a FREE LOAN for a few days, as a matter of convenience - i.e. if you can afford to spend $5K in cash in a week, take a credit line for $5K rather than cash from your own bank account. At the end of the week, if you've lost it all, then you're only where you would have been budget wise if you'd taken cash. And it saves carrying large amounts of cash around too.
But never take a credit line if you lack financial discipline or understanding, or in addition to cash... unless you have VERY GOOD control. Credit lines and alcohol for instance are a very bad combination.