The Britax Frontier is a $270 seat. Not every family out there can afford a Britax Frontier. It's a great seat. We have one for my 5 year old and love it. However, that does not mean that every child under 80 lbs should be in a Britax Frontier. First of all, there are plenty of other seats out there that will keep a child harnessed to booster age. Second of all, booster seats are not inherently unsafe. They are safe and save lives.
I feel like I sound like a broken record on some of these threads but I will say it again. The Britax Frontier has an 80 lb max weight limit with the harness but MOST kids will not stay in the harness until 80 lbs because they will outgrow it by height long before they get anywhere near 80 lbs. Once the shoulders go over the top harness slots the seat is outgrown by height. My dd is of average size and will likely outgrow the seat by height in the next 2 years. With her current growth pattern she will be 54 lbs at that point. No where near the 80 lb max weight limit.
Booster seats are SAFE!!!! If a child is old enough and mature enough to sit properly in a booster seat 100% of the time, meets the height and weight requirements then it is a safe option. A 7 year old is SAFE in a booster seat. There is actually no good info out there that suggests that at his age a harnessed seat is safer than a booster seat.
And to the OP, if your 7 year old does pass the 5 step test, then he is ok with no booster. It is not, however, dangerous for him to be in a booster after he passes the 5 step test (as someone else suggested). A good hbb will offer him some side impact protection that he just won't get in another seat. I still believe that a booster seat, no matter what booster seat, is more comfortable for a 5 year old than no seat at all. If you are worried about her comfort in the booster, look into one with more padding or check out the Air Booster. My dd rides part time in a Sunshine Kids Monterey. It is VERY comfy!