i wouldn't call it useless, even if you can't keep diseases up only with outbreak. the longer you can wait before refreshing diseases the better. if Glyph of Disease pestilence refreshes diseases at 100% on the initial target, you'll be using blood runes for disease upkeep. with reaping as a mastery, you'll probably be trading every other blood rune in for a scourge strike. baseline, you'll probably be forced to use one of the two following rotations: Pest/SS or Pest/BS/SS. use every blood rune for pestilence, and scourge strike with the death rune, or use refresh diseases with a blood rune, use your first death rune for blood strike, and use your second death rune for a scourge strike (if two blood strikes do more damage than one scourge strike, and you aren't gcd capped, it'll be a dps increase to use Pest/BS/SS with your blood/death runes). add disease duration, and you'll be able to do a Pest/SS/BS/SS rotation, or maybe a Pest/BS/SS/BS/SS instead.

If GoD is out, and you refresh blood plague with plague strike (there doesn't seem to be anything better to do with frost runes than IT anyhow), it becomes even easier to consider. you'll be able to use a lot more Scourge Strikes per Plague Strike. with baseline 30 second diseases, you're probably looking at PS/SS/SS on unholy runes. Add 18 seconds to that and you'll probably be able to go PS/SS/SS/SS/SS, trading 2 plague strikes in for 2 scourge strikes every 15 rune refreshes, or about one PS/SS swap per minute.

let's not forget the added wiggle room to the rotation, the increased ease of keeping diseases on multiple targets (through direct PS/IT application) or just letting them tick longer on the boss if you have to do "something else" as a fight mechanic.

i'll acknowledge that it might not be your biggest dps increase per talent point, and might well be the first place we'll look to trim points, but it won't be useless.