If you ask questions on many different topics that stray away from your usual subject matter, it may dilute the ranking of the sub-domain. I think it would be preferable to instead make another subdomain for questions, especially for those whose questions range across many types of topics.
The positive would be if you are a great writer and ask interesting questions, you may increase overall exposure and interest in your hubs and increase traffic to your sites. Having interesting, quality content may override the breadth of topics covered for those who range far and wide. This may neutralize the effect above.
Glad you brought up the topic, it may be something of general interest to members of the HubPages community.