That is why I advise beginners to ignore these video tutorials,because they should be showing both methods.
I have to honestly say that I cant recall an occasion when   the two finger method has impeded my abilty to play chords both legato and staccato.
neither [in my experience] have I found that my abilty to play grace notes in Irish music has been hampered.
Irish music is [imo]linear,and the chording I use is octaves and fifths[similiar to fiddlers double stops],which I achieve with two fingers,if I want to use a grace note I normally cut on the opposite side of the two note chord,with a finger belonging to the opposite hand.
[imo]grace notes are an effect,and in traditional music it matters little whether you double cut for example[an Anote] , b g,or b dor b b
the same applies to single grace notes[I normally do that which is easiest,cut note above or below on opposite side
Rolls, I normally play as a fiddler whould play, without a chord note[So this ornamentation is not affected,by the discussion].
   The occasions,That I do tend to jump fifths[this is different from playing two fingers with one button]is northumbrian music,where the staccato sound [imo]is eminently suitable.
There are occasions where I press two buttons with one finger,but these are a minority.
however, the one finger two button chordal approach is limiting,and used exclusively,prevents you from achieving certain techniques,to teach it exclusively is to teach bad technique.
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