Do you mean "Altec Lansing" instead of Logitech? I can't find a Logitech IMT620.
Your list of pros and cons, plus jdsmoothie's reply sums it up pretty good. I thought about hooking up speakers above my deck, but for the amout of time I'll use them, it's not worth it. If you're out there often, it's a better, more convenient solution, albeit at a higher cost.
It's your money to spend or not, so it's really up to you. If you were to ask: "I have decided I want to hook up speakers to the back of my house, how do I do it and what should I use for my budget of $xx?", then I think you'd find this forum a great place to find people willing to help.
Cablevision - Explorer 8300HD- SARA
Western Digital 250 gb (WD2500KS)