Any reason to not use a 10 gallon primary?

The beer making kit I bought has a 10 gallon primary with a solid lid (no airlock). There is no need for a blow off tube because the large size easily contains the krausen. I've been using it for my 5 gallon batches with no problems however, after searching and reading this forum for a while it seems that everyone uses either a 6.5 gallon carboy or bucket for a primary. It seems like it is a lot easier to use the 10 gallon than it is to mess around with blow off tubes. So is there any reason that I shouldn't be using this 10 gallon bucket as a primary?

There's no reason at all. fermentation will quickly fill all the headspace with CO2.

If you don't have any way for the gas to escape, make sure you don't seal the lid down. Just lay it on top so CO2 can get out. The positive pressure should keep the nasties out. Otherwise it will find a way to escape on its own by blowing the top off which may cause a mess (and lost beer). And even if it didn't, all that CO2 pressure would be bad for the yeast during the initial stages of fermentation.

It would probably be easier to drill yourself a small hole. You don't even need an airlock. Many people just lay some aluminum foil over it.

The lid doesn't snap down like it does on the smaller buckets its actually kind of loose fitting. I think air can get out quite easily which actually concerned me at first.

I don't see any problem, as long as the cat doesn't knock the lid loose and fall in.