The benefits of sticking with one program are obvious (I think).
Are there advantages in being a member of heaps of different programs?
As long as there is no membership fee.

If you fly enough then you can take advantage of special promos for extra miles under the various programs, and also different earn/burn rates. There are threads already on this, for example see www.flyertalk.com/forum/showthread.php?t=184438 or http://www.flyertalk.com/forum/showthread.php?t=184410

Edited. The above is considering different programs within the same alliance. Joining programs on different alliances may increase your access to status benefits and ability to earn/burn (ie can buy more on fare rather than sticking to certain carriers). Again, only of benefit if you fly enough.

Within *A, I do most of my flying on LH, SQ, and UA. I maintain *G in the programs of all three carriers. Why? Two reasons:

1) LH gives more miles than UA for many of my flights e.g. short-haul domestic F on UA earns 1250 miles of which 500 are status miles when credited to UA's M+ but the same flight earns 3750 miles all of which are status miles when credited to LH's M&M. On the other hand, a 1000 mile flight on UAX or busTED would earn 2000 miles of which 1000 are status miles when credited to M+ but only 1250 miles all of which are status miles when credited to M&M.

2) Sometimes the added status helps. Showing three *G cards has worked wonders a few times with grumpy agents as most agents aren't used to pax having more than two. At least I'd like to think so. At the very least, having status on the carrier being flown is better than having *G on a different carrier (except for getting into UA's RCC). SQ in particular takes better care of me as a PPS than as a LH SEN or UA 1K.

I joined the lot,

so ok I may miss out on some of the bonus's they offer for new members but you never know when a card may come in handy!

Dave

In some cases, you will get several more upgrades if you have status with multiple carriers. If I fly 100K on AA and 100K on UA - I will get SWU from both carriers. This can hold true within an alliance too.

I find holding multiple *G cards to get you little benefit (ie waving them to some lounge attendant, cabin person etc) unless of course it is the *G of that airline (this holds particularly true with Thai's ROP *G, they upgrade their *G's first before others on full Y flights) and SK's EB *G - again a better chance of upgrade to economy extra or even biz.

Depending on how many miles you fly, as stated earlier there are clear advantages to clocking to multiple programs. Similarly, if you aren't a methodical thinker, you'll go crazy thinking about all these miles here there and everywhere and the intricacies and changes to each program.. for example did you know miles earned in 2002 in NH's program expire on the 31 Dec 2004 (yes this year....) did you know MX just left *A, potentially making redemptions on other star carriers almost worthless..... did you know LH is about to change earning rates on many economy class fare types... but then again awards are becoming cheaper Aug 1?? It's very very confusing, but if you fly alot, it pays to have multiple memberships.

UA1K top tiers gets you plenty of very nice SWU upgrades.... AC status affords your SWU's in their program and a swag of bonus miles upon reaching mileage tier levels... etc, etc!

So if you do 250,000 miles PA, then it may be worthwhile to pool into more than just 1 program... when weighing up the benefits of each.

Having multiple memberships if you don't fly can also be very valuable. As we've seen with BA's program, they've now restricted memberships to only those who have flown on BA... blowing out the opportunity for many to transfer Starwood Points into the BA program when they ran a 1 SPG point = 2 BA miles! (plus the regular 25% bonus too on 5,000 SPG point transfers).

You'll also be in a prime position to take advantage or be aware of various promotions many programs run from time to time, but this is often dictated by your country of residence sadly (as seen by the UK Comp offer, the various UA US/North American centric promotions recently of Fly 3, 1 free, etc, etc).

I personally have memberships with most airlines, it also pays to sometimes hold those boarding passes and not accrue mileage, and then send in the boarding passes to the most appropriate program at the time (within 6 months). This can prove invaluable if you are short of miles in a certain program which happens to be running a promotion at a certain time, but this too can be a real real pain sometimes. A good example of this is with Malaysian Airlines Enrich Program which was offering awards from SYD-KUL for only 31,500 Enrich Points... I could have clocked my MH flights to ANA's Mileage Club program but I waited and noted this promotion and clocked it to MH's Enrich Program hence getting better value for my miles than I would have got had I put them to ANA's program where the same award is 45,000 miles on ANA.

If you are a prepared to put the time and effort in, yes, multiple memberships can be great, and it always pays to have them as a backup. Whether you use the advantages is up to you...