Totally agree. Makes no sense for it to be always set at 78 when there are circumstances that may come up.....Personally I don't like the preset heaters, as you have no control over your temp. What if you have a species that does better at a little cooler or warmer then 78? What if the preset "78" isn't really 78? What if you need to raise temps for something like ich treatment?
A heater is an electrical piece of equipment that you are submerging in water, there is no reason to go cheap here.
I really like the Aqueon Pro heaters. They have a tough plastic case, so no glass to break. They are fully adjustable. They also have a nice green/red LED system that lets you know when the water has reached the set temp, or if the heater is on and actively heating. I like knowing what's going on in my tanks. For your 10 gallon, the 50 watt version would be perfect. Here is a link from bigalspets.com (buy them online as they are much cheaper then local places).....
Looks like bigals is out of stock currently on that heater, so maybe check kensfish.com or amazon, as they are cheap as well. Bigals will probably be restocked soon, they are pretty good about keeping stuff on the shelves.