Difference between 4 and 5

[nam6641]

the difference between different vbulletin versions 3,4,5 are that whenever you upgrade the vbulletin, You get
1. More server load
2. Slower page Loads
3. Loses Search Engine Rankings each time you upgrade (vbulletin 3 has index.php as default page, vb4 has forum.php/content.php/activity.php as default pages, while vbulletin 5 make a mess for search engines by moving all pages to core directory and change the whole sitemap) And search engines needs to re-index all your pages again after upgrading to vb5. i have read about the two layers system for more security in vb5,i have a simple question that it is good for Search engine rankings to change the site structure so rapidly?
4. You get more new bugs after upgrading.
5. Every time vbulletin upgrades its script the coders have to learn code again from the scratch to make new mods or port old mods to run with new script.
So whats the benefit of buying a vb5. If there is any? please mention thanks,if there are some solid reasons behind the vb5 then i will have no hesitation in purchasing a vb5 license.And one more thing i have noticed that we can't customize the design and templates as we used to do in vb4. is that true?