I don't think the responders here are telling you to diversify exactly, they are telling you to explore your options. Usually the advice on FlyerTalk is almost the opposite of diversify, it is rather to consolidate stays/flights where possible and fiscally responsible, so as to achieve top tier status and maximize earned points and rewards. Research the programs with an eye towards which are represented well in your probable areas of travel. Pick a primary program. Pick a back up program. Stay with the primary when you can and secondary when you can't, but not to the point of silly loyalty in the face of major cost savings.

And as always: never stay in a hotel, fly on a plane, or rent a car without getting credit in the relevant loyalty program, even if it means signing up on the spot.

Some specific comments: I am a Starwood guy, love the properties and love the rewards program. They happen to be well represented in my travel areas, even internationally. But I am able to admit that for generic international travel Hilton and Marriott have way more options. Hilton has 3700 properties, Marriott 3150, while SPG has just under 1000. It can make a difference. I tend to think that the Starwood program has HH and MR beat hands down, but only where they are actually present. Hilton is my backup program, largely for my ability to get midtier status fairly easily.

Once you pick your primary program, you would be well advised to get the branded credit card(s) for that hotel. The SPG AmEx is one of the very well respected CCs around here, and provides great benefits and flexiblity. The HHonors Surpass card gives great point benefits when staying at Hilton properties, and there are several other Hilton branded cards that give lots of bonus points. Chase has a Marriott Visa with 70K points now. So whatever program you pick, get the cards to maximize the benefit, use them especially at their associated property for multi-point bonuses, and pay them off completely each month.