And worst case, he points you to a web site or book that answers your big question, and now you'll know what specific questions to ask. No writer would do their agent the disservice of handing out her name and number to anyone who asks. No writer knows what publisher is best for you. Besides, there's LOTS of info out there to help you pick an agent or publisher that suits you, so asking only implies that you aren't willing to do the work yourself. Asking a random writer to read your manuscript is extremely rude.
Writers often have to work long days to make ends meet.
Or it may be asking them to break a deadline that they can't afford to miss. There are very few people who, when confronted with such a request, would be anything other than angry. If you really, really feel that you have to ask, take a few things into account. One, be polite and ask, don't insist or assume. Two, try asking the writer in question to read a brief excerpt of your manuscript, rather than the whole thing.