Well, I too use the SPG Amex - it's my favorite card. I use a few others as well.

Speaking purely from my own experience, I:
* considered moving some spend to the Merrill+ Visa b/c you'll get 4 Ritz-Carlton nights @ $50k in spend, plus you can redeem the points you earn, etc. I still decided that 50k on the SPG Amex is more valuable... barely. I value the coach tickets you could get with the Merrill card at close to zero.

* I actually did move some spending, just a small amount, to the Hilton Amex -- grocery and restaurant charges, mostly, things that they're giving bonus points for. A credit card company can direct some of my spend via bonus points. The card also offers elite status for $20k in annual spend, but I'm already Hilton Gold so this is of no benefit. I'll probably give them $15k or so.

I have an Amex Plat but I only use it for very large charges -- larger than what I can put on the SPG Amex. I don't use it as my every day card b/c I really don't want Membership Rewards points if I have a better alternative. Still, there are some nice benefits to the card -- lounge access, minor status, better concierge than VIPdesk, the Fine Hotels & Resorts program. It would be my favorite card by far if it earned Starwood points (1 per $1 spent) rather than Membership Rewards points.

My perfect card:
* Earns flexible points, Starwood is my #1 while Diners Club Club Rewards would be #2 (I don't even really mind the transfer fee that much on the latter).

* Provides access to aspirational awards. This may be a subset of #1. I like Starwood points far better than Hilton points because of ease of award redemption and that Starwood has more high-end properties that I really want to stay at. Plus the flexibility of the points means that lots of FTers flew the Concorde before it was retired using SPG points transferred into QF miles.

* Elite status earned via spend. Several airline cards give bonuses towards elite qualifying. Hilton gives you Gold @ $20k in spend. Would sure be nice if someone offered top-level elite based on spend, I'd give Starwood $150k in Amex spend year in year out if they offered Platinum.

* Good customer service, the way Diners used to be.

* Other great benefits: lounge access, Amex FHR program, primary rental car insurance.

Too much to ask? Maybe, which is why I have an SPG Amex, Diners Club, Amex Plat, and Hilton Amex. I also have a UA Visa, mainly b/c I've carried it for about a decade... but other than bonuses for United spend there aren't a whole lot of unique things about it.