It can be good if you use it to guide you, and it can be bad when you let it dictate every little thing you do. I usually use it only for recovery runs (day after a workout), but not really on easy runs (day before a workout) except to look at the average afterwards.
Another poster said your test for your max HR wasn't very good and he's right, after a long run your heart rate can't go as high because you start to go through glycogen depletion (probably not 100% for anyone really), which lowers your max for a bit. I found mine (200) by wearing it during a 5k and the saw the peak when I kicked (it was 195 ish for a while and then I hit 200 at the end) which also isn't perfect, but very close since running at VO2 max or a little faster for 5 - 6 minutes with a proper warm up should bring out your max HR.
As for 70% being that slow, you don't give an indication of your other PRs. I tend to train at 70% HR for my recovery runs and that's usually about 7:15 - 7:45 for me (min/mi), but I'm in the 15s for 5k, so that makes sense. If you're a 20+ min 5ker, sometimes you might need to go pretty slow if you're tired, but if you're sub 20, that seems like either your max is higher (likely) or your HR monitor is off (also likely if you're using a wrist based monitor). When doing easy runs (so not when you need to recover, but are just doing miles between workouts), you can get as high as 80% and still be fine.
One last thing, think about how much mileage you're doing when using heart rate. If you are running relatively low mileage (say less than 40), you can go a little faster and still recover (so more like 80%), but if you're running high mileage (say 90+ mpw), then it makes more sense to take your easy days a tad easier at say 78% max.
Take from that what you want