There is no good way to subscribe to multiple similar tags with one email subscription. For example if you wanted email alerts for both asp.net-mvc and asp.net-mvc-3 you would have to create two separate alerts and there would be a lot of questions duplicated between the two similar tags because those questions are often tagged with both. It would be great if you could subscribe to multiple tags on one email alert to cut down on the duplicate questions being asked.
You can (and should) use a filter to follow multiple tags. This blog post describes filters in more detail, but basically they allow you to follow sets of tags on sets of sites through the filter page, RSS, and email.
If you don't see the "Email Updates" box as shown here:
Perhaps one of these, such as this one which follows asp.net-mvc, asp.net-mvc-2, asp.net-mvc-3, and asp.net-mvc-routing, on all sites, would work well for you? If not, feel free to create your own filter.
I think it would be better to remove any duplicates from every bundle of email alerts that is about to be sent. Say that a user has subscribed to two tags and there's a question tagged with both. Now the system will collect the same question twice and send two email alerts. What should happen instead is that the system should check for duplicates before sending email alerts. This does what you want without the need to implement a new feature for users.