Acid Blast and Windstorm are ranged so you can still use them that way and not necessarily target self / friendlies.
So its late game I have my [mwcard=MW1C20]Highland Unicorn[/mwcard] who after Upkeep has one Life left and large stack of Enchantments that I would love dearly love to not loose. The only other creature in the zone is [mwcard=MW1C07]Cervere, The Forest Shadow[/mwcard] who has two Life left. I am one zone away but my opponent has initiative the only thing I have left with range is you guessed it: Windstorm. Luckily it is Unavoidable but now I have to target Cervere and my Unicorn, Cervere lives and the Unicorn dies.
Thanks a lot Geverus Tnape at Slogwarts for not teaching me how to aim my sweeping spells so I don't have to target my friendly creatures but I can catch that stoopid flying golf ball! You would think that after a 1000 years of magic school they would teach this most basic of skills!
No different than only having a zone attack left
Zone attack targets and entire zone, a sweep attack targets two different creatures, double / triple strike target the same creature multiple times. While a bit silly logically, I agree, there are only a small percentage of times where this would be an issue. Sweeping attacks are generally less effective than the creatures quick action options. Selesius and war sledge are the current exceptions to that statement, and Selesius cannot target herself anyway. War Sledge loses the ability to single target daze.
Most other situations are moments where the player does it to themselves, no different than targeting a zone with friendly and enemy creatures with a zone attack. The above DI example is a once a game situation but can happen.
Mind control is also worth mentioning as the creature can attack an enemy creature and himself; which is a win win for the forcemaster.
Also something to note is if the sweeping creature does attack himself, it would also trigger his bloodthirsty bonus dice trait if it had one.