I am SHOCKED at such bad advice coming from a supposed car seat tech/fireman. He must have forgotten what he learned in his certification classes! The bare minimum for safe booster use is 4 years AND 40lbs. She doesn't meet either requirement. And, experts recommend keeping kids in a 5 point harness AS LONG AS POSSIBLE. So she is definitely better off in a car seat with straps. I would actually be a little surprised if she was truly outgrowing the Marathon at only 3.5 years and 38lbs. Remember the Marathon goes to 65lbs (except in Canada - 60lbs) and the top slot height is 17". My 95% height/95th% weight child just outgrew his Marathon at 45" tall and 44lbs. As for it not being able to hold her, rest assured, as long as she is less than 65lbs and her shoulders are at or below the top slot, she is safe. That seat won't expire until 2010. Are you sure you have the Marathon, or did you get the smaller Britax seat, the Roundabout. If you have the Roundabout, I can definitely guess she's outgrown it. It only goes to 40lbs.

WHY KEEP THEM HARNESSED:
Compare the following crash tests for your self and see the major difference a harness makes.
Forward facing harness vs. booster: http://www.oeamtc.at/netautor/html_seiten/kisitest_2002/videos/test2002/frontcrash/recarostart.mpg
A lateral test (the third view in) of a harnessed seat: http://www.oeamtc.at/videobox/index.php?domain=oeamtc&videobox=Kindersitze_bis_18_kg_oder_4_Jahre&movie_kind=swf
A deep winged high back booster like a Parkway: http://www.oeamtc.at/netautor/html_seiten/kisitest_2002/videos/test2002/seitencrash/concordliftpro.mpg
A shallow winged high back booster like the Turbobooster: http://www.oeamtc.at/netautor/html_seiten/kisitest_2002/videos/test2002/seitencrash/htsbesafe.mpg
And a no back booster: http://www.oeamtc.at/netautor/html_seiten/kisitest/videos/tests_2001/seitencrash/Baby_Sunny_touring_P3_vorne.mpg
Crash test video showing a 5-point harnessed seat next to a belt positioning booster
http://nz.youtube.com/watch?v=NxfIuhL3cXk&feature=related
VIEDOES OF KIDS WHOSE PARENTS DESPERATELY WISH THEY'D KEPT THEM HARNESSED:
http://www.youtube.com/watch?v=azgBhZfcqaQ
http://www.youtube.com/watch?v=V2kO8AxKbrM
http://www.youtube.com/watch?v=KPBSEQ4xbTw
We also now know the spine is not solid until age 6.

So to sit in a booster and use just the seatbelt like an adult (basically a booster just fits the belt to the child,so it's essentially the same), they need to be able to not have the lapbelt cause those injuries, have the head proportionate like an adult, and have a solid spine like an adult. kids under age 5 have higher rates of head injury if they are riding in boosters than kids over age 5 (presumably because younger kids have heavier/larger heads in proportion to their bodies, so they fly forward farther or with greater force, thus incurring greater injury if/when they come in contact with vehicle interior or intrusion?).

"Another type of misuse is a child not using the restraint designed for its size and age. The
study of Isaksson-Hellman et al. (1997) showed
that the maximum effect of a restraint system is
not attained if the child is not using the optimal
CRS for its age. Also, a tendency of higher
injury risk was identified when the growing
child switches from one restraint to another, i.e.
when the child is at the youngest age
recommended for the restraint. " -SAFETY FOR THE GROWING CHILD – EXPERIENCES FROM SWEDISH ACCIDENT DATA
Lotta Jakobsson
Irene Isaksson-Hellman
Björn Lundell
Volvo Car Corporation
Sweden

http://www.carseat.org/Boosters/630.htm
http://www.boosterseat.gov/
http://www.cpsafety.com/articles/BoosterShop.aspx
http://www.car-seat.org/showthread.php?p=303866#post303866

Even if she does have a super long torso and has about outgrown the harness in the Marathon, there ARE other options that will keep her much safer than a booster seat. The Britax Regent is one. It will keep her harnessed longer than any other seat on the market - going to 80lbs and the top slots are 20". Even my big boy has plenty of room to grow in it. It retails for $269, but can often be found cheaper online. Try amazon.com (just make sure its new and not used) albeebaby.com babyage.com and babyuniverse.com If you sign up for free with ebates.com, you get 4% cash back on purchase from babyage.com It is one awesome seat- keeping the average child harnessed to age 8, and hopefully keeping my big boy harnessed to at least 5-6 years.

Another option - Graco Nautilus, at only $150 this seat is an awesome deal. Harnesses to 65lbs, with top slots of 18-19" (the seat is really new, we're not sure yet!) but my son has the same amount of growing room in it that he has in his Regent. And then it beomces a booster and then a backless booster. GREAT seat! A little hard to find since they just came out and are incredibly popular. Check amazon.com, Walmart supercenters (the regular Walmart stores don't seem to carry them) and Burlington Coat Factory. You can also see walmart.com and choose site to store to have it shipped for free.

A third and final option is the Safety1st Apex65/Cosco Alpha Elite. But its not nearly as great as the Graco Nautilus, and isn't a lot cheaper. Your vehicle's headrests MUST go up to the middle of the Apex65 headrests in its highest position to use the seat. The Apex65/Alpha Elite/ Signature BIltmore has no support in the back of the seat, so it has to be supported by the vehicle's seatback. Also, there've been complaints about the pointy buckle, as well, and its a little more difficult to install. But it is slightly cheaper.

And talking to a CPST is a great idea, but please do make sure the person really is a certified child passenger safety technician. Not every fireman and police man are car seat techs, and they have to renew their certification every so often.