My luck,I'd patch it,and within 6 months I'd have some sort of storm damage and end up replacing the entire roof.
ok ,ok,I'll be more serious...
Being in the middle of the country,living in an old house,my thoughts are probably different. Severe storms,and ice storms in the winter (2 -3 in the past 15-20 yrs) plays a factor.Around here it's common practice to leave the old roof on,and put a new layer of shingles over them,so a roof might have 3-4 layers of shingles.How many trees are near the house,that could cause damage to the roof.So your weather,and things it's had to deal with the in the past 10 yrs might affect your decision.
I'd replace the whole roof,If it was an older house,and I was really happy with the house that I planned to live in for at least the next 10-15yrs. Chances being I'm going to have to replace the whole roof in that time period.Most shingles life 20-30yrs,especially on older houses,where the roof was probably replaced at some time.
Newer house,might have better products (better water proof membranes,instead of old felt paper),less chance of "other" damage not seen by other events.