Are diplomacy ships even useful?

They are very useful, just hard to get and maintain in a multiplayer game; most games you are better off focusing on your opponent then getting pacts. Generally the only time I get pacts is when I am TEC and in eco position with someone else who will benefit from the pacts also.

Cost of Pacts:

1. Labs (Money and Logistic slots)

2. Envoys (Factory time, Money and Fleet supply)

3. Tech (Money)

4. Time (In the short term ships will give you a getter ROI)

5. New Vulnerability (ever been at -600 fleet supply after you lost an envoy and supply pact?)

It really depends on the specific game on whether or not pacts are worth it for example if you have just one roid having every pact from all races wont help much; while on the other hand if you have 10 desert filled with trade and a long trade line, trade pact might net you 50+ credits a sec giving you a great reason to get it.

i dont play online there very good in sp though

TEC and Vasari envoys are good if you have the right abilities for them (mainly the ones that increase trade and resource income)...

One thing I'd recommend is to manually have your envoy target the planet once it has reached the gravity well...this will cause the envoy to stick by the planet...if you don't do this, the autocast for some reason has the ship retreat to the edge of the gravity well after using an ability, and only come back to the planet after the cooldown, wasting valuable time...

Partial solution to that is to order the Envoy to move to the planet itself (as in right click the planet).

That's what i was getting at, guess I did not word it real well...why envoys and colony ships do this is beyond me, it would seem they went out of their way to program this behavior when it only makes things worse...

You're right.  In online games people rarely build Envoys, and/or get Pacts.  Why?  For all the reasons listed:  money, time, resources, envoys (fleet supply), labs (logistic slots), research, etcetera, etc..

But I think the biggest reason is the ridiculous 12 fleet supply that each Envoy consumes.  And it takes too many Envoys, or an enormous amount of time to raise relations to a point that you can actually get pacts.  Especially the supply pact which you mention (8 civic labs and 17.5 relations)!

A good online feeder (eco player) only has 50 supply in excess (at the standard Normal Fleet size setting).  Therefore, he can hardly afford to use about half of his remaining supply for a couple Envoys.  He needs to reserve some for scouts, and colony ships for capturing neutrals, as well as his Cap ship and support.  A couple Envoys will barely raise relations at all. 

If you increase fleet capacity you suffer increased fleet tax (+9.5%) on all income, and subsequent feed is also taxed at the higher rate. 

The researchable techs for Envoys are probably a better deal than pacts, but nobody seems to use them either.  In particular, the tier 4 Vasari tech that essentially doubles a planet's extractor output and it's factorys' ship production rate while an envoy is present, is quite valuable and applicable to almost all planets.   The tier 3 TEC tech that increases the envoy's planet's trade by 50% 1/3 of the time can be very valuable, especially if it is a desert loaded with tradeports, but rarely will a military focused teammate have that type of planet present.  Also, the tier 2 TEC "Worthy Cause" tech that 'donates' $60 every 30 seconds is nice and usually worthy.  Advent's tier 3 envoy culture tech is crap, IMO, and also rarely applicable.  All of the remaining higher tier Envoy techs are also all generally poor investments, and/or very situational.  

IMO, Envoys should kick back something (maybe a portion) back to the owner as well, (beside a measly .10 relations boost with slow relations growth).  Also, I would like to see Envoys having some form of small military value.