I think that charging $2.99 a month will bring in new subscribers. Here's why I say this: Currently, to listen to the free internet feed one needs to put in their user name and password - then there is no extra charge to listen to Sirius XM on the computer. I am sure there are some people who simply give out their user name and password to a friend, and then that friend can listen at work or at home for free. In effect, two subscriptions are in use for the price of one.
Soon, with the $2.99 per month charge, it stands to reason that not all of these "generous" people will want to pay that amount if they themselves do not listen on the computer. So, if their freeloading friends want to continue listening on the computer, they will have to pay either $12.99 per month to listen on their computer (if they do not otherwise subscribe); or they will have to take out radio subsciptions ($6.99 per month fee or above) and then pay the $2.99 internet fee - yielding a minimum of a total of $9.98 per month from each of those people.
Of course, there are probably not a huge number of people for which the above applies, but perhaps there are enough to generate a modest revenue increase for Sirius XM and increase the subscriber numbers a little.