Based on this position statement from the American College of Sports Medicine, the answer to both questions is yes.
According to the ACSM, sets of 15-25 reps (using light loads) are most effective for increasing muscular endurance, and have also been shown to increase strength in moderately trained people. High rep sets may also supplement a conventional training programs:
Goto et al. showed that the addition
of one set per exercise (to a
conventional RT workout) consisting of
light loading for 25-35 repetitions
led to increased muscle CSA [cross
sectional area] whereas conventional
strength training alone (e.g.,
multiple sets of 3-5 RM) did not
increase muscle CSA. The addition of
the high-volume sets led to greater
acute elevations in GH [growth
hormone]. However, light loading alone
may not be sufficient as Campos et al.
have reported that 8 wk of training
with two sets of 25-28 RM did not
result in Type I or Type II muscle
fiber hypertrophy.
Incorporating some low-rep sets is recommended for advanced hypertrophy (bulking up) training:
For advanced training, it is
recommended that a loading range of
70-100% of 1 RM be used for 1-12
repetitions per set for three to six
sets per exercise in periodized manner
such that the majority of training is
devoted to 6-12 RM and less training
devoted to 1-6 RM loading
If you're doing power training (training to apply strength quickly; important for many sports), you should incorporate some faster-paced sets. These sets should have relatively low reps:
It is recommended that concurrent to a
typical strength training program, a
power component is incorporated
consisting of one to three sets per
exercise using light to moderate
loading ... for three to six
repetitions