I don't find trial factoring very interesting in itself, the results are too predictable, I need to have some interest in the numbers being factored. The only thing I can see that is interesting about an arbitrary billion digit Mersenne number is the possibility that it might be prime, but there is no way to test for that yet, and probably won't be for a long time. With a number like MM61 it would also interesting to discover that it is composite, so it doesn't matter that a primality test is not yet possible.