I prefer metal finger picks for playing acoustic guitar - a steel finer pick can be bent a little so that the tip of the pick corresponds with the contact point of a naked finger hitting a string. This indicates that there's no realignment required for the angle of attack. Plastic finger picks are commonly thicker and protrude a bit more, so an adjustment in picking technique is necessary. This is essential if playing some songs that need picks, and some that don't, for example.
Thumb-pick opposed to bare thumb.
Generally, the pick provides a sharper, louder sound and amplifies somewhat. Some striking techniques are less difficult to accomplish in this way, such as 'throwing' the thumb onto the string giving a percussive effect. A significant benefit is that it saves the thumb from becoming sore. Bare finger guitarists have to practice very routinely to grow a thick callous so that they can play in comfort. In general, picks are easier to get in between the strings, so are great for picking individual strings in rapid succession.
Finger Picking With Bare Fingers.
A naked thumb can hook behind a string, pull it up from the sound board of the guitar and let it snap back, generating an accented beat which can be attractive in some styles of blues. In general, bare fingers are really precise and can create a wonderful bouncy style when performing ragtime guitar, either in the style of Scott Joplin-type piano rags or original ragtime blues in the style of Blind Blake. The thumb rolls and triplets highlighted in the latter's music make it really hard to play with finger picks.
Many of the latter stylists, similar to Chet Atkins, utilized strengthened finger nails. I doubt if any older blues guitarists utilized this technique, but who can say? A lot of Texas guitarists favored a plastic thumb pick and bare fingers, frequently just using one finger of the right hand with incredible dexterity. Blind Blake seemed to have used naked fingers and and some contemporary blues men recollected that he had a hole in his right hand thumb, exactly where it was worn down by the bass strings of his guitar.