This is true, especially if you are singing lead. Like Saving Abel's "Addicted" and Velvet Revolver's "Slither" come to mind. Being able to hit that open D makes things waaaayyyyyy more manageable.
I was referring to those 4 string players that seem to think by playing a 5 you cannot tune the e to drop d. Now a lot of drop d songs I can easily play in standard beadg but some songs are easier to play bdadg. For example I can play several Tool and RATM songs just fine in beadg but some of those bands other songs I can play decently but loose some of the groove because of the original tuning of the song. So they are easier to play in drop d. People seem to forget 5 strings have everything a 4 string has. The same can't be said for the opposite. Now yeah not all music calls for a low b string , but most modern music ranging from country to metal does make use of the low b string. That's not to say you can't tune a 4 string down, but then you lose the g string