Playbalance can be done mathematically one of two ways; both are valid viewpoints: average, and median. You can also look at some more complicated metrics, but they're typically designed to account for human observational inadequacies, such as Beyesian Averages (look to BoardGameGeek for a great explanation as to what a Beyesian Average is). Let's look at both of these with the change in proposals, and see why the change above is actually an improvement on beams.
Average: The average of a set of outcomes i elements of N is defined as: (sorry if the ASCII art doesn't come out right:
....N
.(Sum) value/N
....i
I think that we agree that the change in the mechanic involved does not update the average --- it started at 1, and it's staying at one.
Median: The median of a set of outcomes, i element of the universe of results, N is done by ordering the values of the i events in an ascending order, and selecting the N/2th element, or averaging (N-1)/2 and (N+1)/2 if the cardinality of N is even. Here, the proofs get a little harder.
The median for the new method we can do by exhaustion. There are 36 outcomes: 1-1, 1-2, 1-3, 2-1, 2-2, 2-3, 3-1, 3-2, 3-3, 4-1, 4-2, 4-3, 4-4, 4-5, 4-6, 5-1, 5-2, 5-3, 5-4, 5-5, 5-6, 1-4, 1-5, 2-4, 2-5, 3-4, 3-5, 6-1, 6-2, 6-3, 6-4, 6-5, 6-6, 1-6, 2-6, 3-6. The values are:
0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2
The average of 1 and 1 is 1 --- so the median of the new method is 1.
The median for the old method is more difficult, but there are an infinite set of possibilities. However, it can still be calculated. Consider all possible sets of infinite sequences of die rolls. We need to find the middle pair of the sequence.
First of all, we must establish that number of these sequences is even. This is easy; we can claim that the number of sequences is divisible evenly by 2 by example. Separate the series of infinite sequences in 2 subsets, those beginning with a 1, 2, or 3, and those beginning with a 4, 5, or 6. For every sequence that starts with 1, consider an exact same sequence starting with a 4. There exists 1 and only 1 such sequence. The same relationship holds between sequences starting with 2 and 5, and sequences starting with 3 and 6. By implication, if there is a 1-to-1 and onto mapping between the two sets, they must be isomorphic, and isomorphic sets have the same cardinality. If we can divide one set into two subsets of equal cardinality, then the set must be even.
Second, note that every die roll sequence in the old system that starts with a 1, 2, or 3 will result in zero hits. In fact, while the infinite sequence of die rolls may exist, no one ever rolls them; they don't matter.
Now note that if we ordered all the results in ascending order, the (N-1)/2the element would result in zero hits. As the cardinality of the set is even, we must find the (N+1)/2th element and the number of hits it has.
This (N+1)/2th hit will be the lowest number of hits in the sequences that start with 4, 5, or 6. I will note that, at worst (so, yeah ... my proof is sucky) we can consider the number of hits of any sequence that starts 4,1,........ This will result in 1 hits.
So, the median of the old method for rolling beams is the average of the (N-1)/2th element (0) and the (N+1)/2th element (1). The old median is 0.5.
Old Median: 0.5
New Median: 1.
If you balance by average, nothing has changed. If you balance by Median, beams have improved.
You can also consider balance by Mode, the most common outcome. I think this method is crap, and most designers will, too, but you can do it --- it only helps my argument!
The Mode is the result that is most common. By exhaustion, above, the Mode of the proposed beam mechanic is 1. Again, it is slightly harder to get the mode of the old mechanic, as the number of results is infinite, but I hope I won't have to write a full proof about why the Mode of the old sequence is 0.
Old Mode: 0
New Mode: 1.
At least, by the major balancing statistics, beams have actually improved. Now, there are other concerns that you might have -- harder to get explosions to cause collateral damage, perhaps -- but I will remind beam users that one of the reasons I overheard that the Whitestar was given a second die of beam is that the first die was too unreliable. So, reliability and predictability are valuable themselves; apparently more than extreme results. Arguing that, and then arguing that you should still have the ability to "roll up", means that you are simply arguing for a beam upgrade in general. Which is it to be?
I think that KNOWING I'm going to get some hits is valuable. Maybe I don't want to explode that ship all over my fightercraft, for example. I've seen my opponents lose a lot of fighters due to their own overeager beam fire, and predictability would be valuable there.