Toei lines: are the missed?

If I get a bunch of those 2-day Tokyo Metro tickets, how much if hinderance not being able to use the Toei lines would be?

It is less the Toei lines and more than JR lines (especially the Yamanote Line) which you are probably going to miss dearly if trying to use Tokyo Metro passes exclusively.

I figured as much, but are the savings really worth it - 600 jpy if I get them at Narita, if I remember correctly...

I mean, 25 vs 30 minutes to get somewhere won't make a difference, but if it's twice as much or so...

You don't need the Yamanote. Sure, travel times will be longer on the Metro, and you may have to do a bit of walking, but all day metro passes are great price wise in comparison to using the Yamonote if you intend to travel to more than 3 or 4 places. There's also a lot of places the Yamanote can't take you.

What price do you put on convenience ? We pay thousands of dollars to visit a foreign country and then carry on like Scrooge trying to figure out how to save ten. Get a SUICA card and do it the quickest and most convenient way. Too much to see and no time to spare.

Well said RodW: airfares and accommodation vs a few 100Y??
Suica for convenience every time.
If you have a JR Pass use it on the Yamanote, and suica on the subways etc

I can understand why they would want to save money, some people are really on a budget, other travelers' are bean counters like myself.

Now I live in Tokyo, but every chance I get I look for ways to save money, even if it's 100yen here or there.

Tokyo Metro can get you to MOST places in Tokyo just fine and places JR doesn't even go, it depends on where you are starting and where you are going, however the time difference is not THAT great, it can be even faster then JR at times.
So a few extra minutes and a few extra yen saved. Thats good in my book.