Haha where would you like me to begin?
Pros:
I can hit H2R one day, or even half of a day, and then go STRAIGHT to the motocross track and ride it competently without changing a thing. Just add fuel!
The street legal ones are DIRT CHEAP to insure, and get decent gas mileage if ride them right.
They use smaller tires which COULD mean cheaper tires.
Depending on what bike you get, you might have to change the oil(s) often, but again, model dependent some last just like regular streetbikes.
Cons:
The maintenance of a dirtbike converted motard is more than a streetbike because they are generally setup from the factory for hard motocross riding, thus require the attention to keep it running tip top.
Small gas tank - even street models have a small 2.5/3gal tank, of course you can always buy the bigger enduro style tanks, but I think they look stupid.
They're so fun you can get into trouble or so I've been told. I don't really hang around for the cops to find out what kinda laws I've broken.
I LOVE my converted Honda dirtbike motard. It's a pure offroad/track bike. Which is what I wanted. The downside is I can't take it on the street, but I rarely rode street with my ninja anyway so who cares. It handles great on the asphalt and after really testing it on the MX track, it seems to handles jumps and dirt riding just fine too. I couldn't ask for more, well I could but I'd just be getting greedy.
Even though there's lots of "motards" out there, you need to decide what you'll use it most for and go from there. There's some factory motards(Suzuki DRZ400 & Yamaha WR250X) which are great street/"hooligan" bikes, but I probably would have no issue destroying their suspension at the MX track. And likewise if I were to take my motard out on the street, I'd run out of gas within a few miles and/or require an oil change after my venture. One option I would DEFINITELY recommend for YOU, is getting one with electric start. Lori has a bitch of a time kicking mine over, and I'm sure even though you're femur is broken on the non-kick leg, it can't be fun. LOTS of them come with E-start, however the "Big 4" Japanese makers full-MX bikes are kick only, HOWEVER their trail bikes come with electric start and full lights. KTMs and euro bikes most generally have E-start including their MX bikes aside from a few models. I'm sure there's more I missed, but I'll post up some more when I recall it.